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Sure this has been debated before, but can't find any posts when searching.

I mean, I know they can take command of Imperial Navy ships and regiments (but how much? can they say "i take command of this superbig fleet or all Astra Militarum forces in this sector")

Can they go to Macragge and order the Ultramarines around? "like hey i take command of this company or even Chapter, I want you to do this and that"

Can Hector Rex or someone like him order the Custodes around?

The answer is however much people allow them to have, which is a matter of politics.

In theory, by the letter of the law, any Inquisitor, even the freshest newly minted Inquisitor, is answerable to no one except the Emperor. In theory they can commandeer the fleets of an entire Segmentum and demand the arrest of all the High Lords. In theory a single Inquisitor commands enough power to crash the Imperium or take it over completely.

In practice, anyone trying to do so is going to find they get denounced by other Inquisitors and other similarly powerful peers of the Imperium as either insane or corrupt, and then be disappeared or have a lethal accident. In practice, an Inquisitor is still going to need to "work the system" and use some political skill and finesse to get things done.

As above? Pretty much whatever they think they can get away with.

The greatest limit on their powers is the Inquisition itself. Not necessarily because of design, but because of good old fashioned human ego and hubris.

In terms of the various Imperial Forces, it’s really only Ad Mech and Marines that are autonomous enough to refuse assistance.

SoB are probably next - insofar as it’ll depend on the Inquisitor. Pious Puritan Inquisitors wouldn’t face much, if any, resistance there. Radicals? Not so much.

Best thing is to read the Eisenhorn stories as they and then the Ravenor series give you a great insight into the world of the Inquisitoin.


In general their power varies considerably and it also depends how they choose and like to operate. Some Inquisitors are more martial and will command capital ships, armies and generally take a more hard handed military angle; others take a much more stealthy approach and might only have their close retinue of staff to hand in any one encounter.


It also depends on their rank within the Inquisition; lower ranking Inquisitors who also don't play the political game enough might, on paper, have a lot of power, but could find themselves not as respected and cut short. Meanwhile higher ranking officials will be able to wield far more significant power.


Plus don't forget scale comes into this, many Inquisitors will have a "home patch" they operate within; places where they own properties, have access to banks and resources; staff, agents and in general a network of operations. Within their home territory they might wield more political power; outside of it they might have to rely on others.




Automatically Appended Next Post:

 Mad Doc Grotsnik wrote:

SoB are probably next - insofar as it’ll depend on the Inquisitor. Pious Puritan Inquisitors wouldn’t face much, if any, resistance there. Radicals? Not so much.

Ahh I forgot about that aspect, yes even as Inquisitors take different pathways toward their goals some are more Pious and puritan - sticking to the letter of the law and the word of the Emperor. Meanwhile others are more likely to dabble in xenos artifacts, psychic powers and even items of chaos. Whilst they have right by the Emperor himself to do as they please in his name, such Radicals can sometimes find themselves lacking in certain political allies. An Inquisitor making extensive use of Xenos services and artifacts might find they have to hide such activities even from others of their own Inquisition; or from select groups they want to curry favour with. Meanwhile a Puritan one might find allies in the Sisters of Battle, but their inflexible attitude might find themselves at odds with the reality of a galactic empire - perhaps bringing down the wrath of the Emperor upon a world that has the only crime in using Xenos technology to mitigate local environmental risks. etc...

Totto wrote:
Sure this has been debated before, but can't find any posts when searching.

I mean, I know they can take command of Imperial Navy ships and regiments (but how much? can they say "i take command of this superbig fleet or all Astra Militarum forces in this sector")

Can they go to Macragge and order the Ultramarines around? "like hey i take command of this company or even Chapter, I want you to do this and that"

Can Hector Rex or someone like him order the Custodes around?

As the others have said, the biggest limit is other Inquisitiors, as they will start asking what, exactly, get needs all these forces for?

When it comes to the space marines and custodes, the inquisition has no direct authority. The marines are ALSO answerable only to the god emperor himself, so he can't just walk up and order a chapter to follow him.

However, they wouldn't just refuse outright, but question him about what he feels needs the marines to do. And, more often than not, go do what they were asked to, because the inquisition exists for a reason and faces threats that might well need a full chapter to deal with

Oh, and I’m pretty sure Cutodes have absolute Carte Blanche to tell everyone, up to and including the High Lords “no, bugger off”.

Sisters of Silence too, possibly? Not terribly up on their background.

They can, but they need to write it down...

Space marines can't be ordered by an Inquisitor but they can be requested. Keep in mind there is political pressure that can often make chapters more co-operative.

Imperial Guard can be ordered. They can take over an army pretty much whenever they feel like. One took over the siege of Vraks half near the end of the siege.

Assassins- Same as space marines.

Nobody can order Custodes as far as I am aware. Not even the High Lords of Terra.

Inquisitors can order marines to comply if I remember correctly, however that may also bring issues for the inquisitor, especially with more independent chapters, I'm sure many inquisitors may well have disappeared in such circumstances.

I think it is much more common to request a chapters help, and well, an inquisitor requesting is going to be more likely of gaining it than a planetary governor requesting for example.

Also of note, inquisitors have full authority to investigate and sanction marine chapters also, though again, it would probably be more likely an investigation then a council of chapter masters would reside over the sanction to be actioned (unless it is full blown heresy etc obviously).

Effectively, just like real world upper echelons of politics, power and intelligence organisations, what is written down as a rule and could be followed to the letter usually involves a lot more compromise, and behind closed doors deals than it should.

Even for high level puritans they have to be careful in how they approach marines for help, especially when dealing with first founding chapters, you could very quickly be black listed as an ally by not just those, but all their successors also.

"My patience is limited, unlike my authority."- Gregor Eisenhorn.

All the power they like, basically.

Rebel4ever85 wrote:
Space marines can't be ordered by an Inquisitor but they can be requested. Keep in mind there is political pressure that can often make chapters more co-operative.

Imperial Guard can be ordered. They can take over an army pretty much whenever they feel like. One took over the siege of Vraks half near the end of the siege.

Assassins- Same as space marines.

Nobody can order Custodes as far as I am aware. Not even the High Lords of Terra.

Marines can and often are ordered to do what an Inquisitor says - whether they do so. like any powerful organisation will depend on mnay things;

What is asked
Who the Inquisitor is and his or her influence
The Status of the Chapter and their own friends or enemies in the Inquisiton
If the task is suitable
If the resources are available.
Who is being asked - if a Inquisitor turns up somethwhere and a small Astartes force is told to aid him, its highly unlikely they will refuse

Like anyone else with that level of power - a Chapter Master (Or Arch Magos, Planetary Governor, Fleet Admiral, Canonness etc ) would have to have a good reason to refuse aid - and usually why would they? If they have concerns about the Inquistors "request" then the really powerful ones may well be able to contact another Inquisitor ally who they have aided in the past - another reason NOT to refuse them without a really good reason is that they may be useful in the future.

If he or she is refused, the Inquistor often has limited immediate options.

The marines are ALSO answerable only to the god emperor himself, so he can't just walk up and order a chapter to follow him.

Thats just wrong.

There are the High Lords and of course the living Primarch as well as the Inquisition

 Mr Morden wrote:

Rebel4ever85 wrote:
Space marines can't be ordered by an Inquisitor but they can be requested. Keep in mind there is political pressure that can often make chapters more co-operative.

Imperial Guard can be ordered. They can take over an army pretty much whenever they feel like. One took over the siege of Vraks half near the end of the siege.

Assassins- Same as space marines.

Nobody can order Custodes as far as I am aware. Not even the High Lords of Terra.

Marines can and often are ordered to do what an Inquisitor says - whether they do so. like any powerful organisation will depend on mnay things;

What is asked
Who the Inquisitor is and his or her influence
The Status of the Chapter and their own friends or enemies in the Inquisiton
If the task is suitable
If the resources are available.
Who is being asked - if a Inquisitor turns up somethwhere and a small Astartes force is told to aid him, its highly unlikely they will refuse

Like anyone else with that level of power - a Chapter Master (Or Arch Magos, Planetary Governor, Fleet Admiral, Canonness etc ) would have to have a good reason to refuse aid - and usually why would they? If they have concerns about the Inquistors "request" then the really powerful ones may well be able to contact another Inquisitor ally who they have aided in the past - another reason NOT to refuse them without a really good reason is that they may be useful in the future.

If he or she is refused, the Inquistor often has limited immediate options.

The marines are ALSO answerable only to the god emperor himself, so he can't just walk up and order a chapter to follow him.

Thats just wrong.

There are the High Lords and of course the living Primarch as well as the Inquisition

and in each of thsoe examples while they have considerable influence they cannot COMMAND a space marine chapter. when the high lords want a space marine chapter to do something they REQUEST it. that applies to gulliman as well

BrianDavion wrote:

 Mr Morden wrote:

Rebel4ever85 wrote:
Space marines can't be ordered by an Inquisitor but they can be requested. Keep in mind there is political pressure that can often make chapters more co-operative.

Imperial Guard can be ordered. They can take over an army pretty much whenever they feel like. One took over the siege of Vraks half near the end of the siege.

Assassins- Same as space marines.

Nobody can order Custodes as far as I am aware. Not even the High Lords of Terra.

Marines can and often are ordered to do what an Inquisitor says - whether they do so. like any powerful organisation will depend on mnay things;

What is asked
Who the Inquisitor is and his or her influence
The Status of the Chapter and their own friends or enemies in the Inquisiton
If the task is suitable
If the resources are available.
Who is being asked - if a Inquisitor turns up somethwhere and a small Astartes force is told to aid him, its highly unlikely they will refuse

Like anyone else with that level of power - a Chapter Master (Or Arch Magos, Planetary Governor, Fleet Admiral, Canonness etc ) would have to have a good reason to refuse aid - and usually why would they? If they have concerns about the Inquistors "request" then the really powerful ones may well be able to contact another Inquisitor ally who they have aided in the past - another reason NOT to refuse them without a really good reason is that they may be useful in the future.

If he or she is refused, the Inquistor often has limited immediate options.

The marines are ALSO answerable only to the god emperor himself, so he can't just walk up and order a chapter to follow him.

Thats just wrong.

There are the High Lords and of course the living Primarch as well as the Inquisition

and in each of thsoe examples while they have considerable influence they cannot COMMAND a space marine chapter. when the high lords want a space marine chapter to do something they REQUEST it. that applies to gulliman as well

Yes they Can - happens all the time in the Lore. same as the Inquisiton - if they term it as a request - thats being polite

But the point is its still a request, the Marines can turn them down. It's basically a huge political dance. On paper they can do certain things, but the reality of the situation is that the paper is only as strong as the politics behind it and with the Emperor in limbo the powers behind rules and laws he put into place are in the hands of others.

And yes at times someone will demand something to the letter of the law and others will follow; perhaps just because they see value in the end goal and aren't looking to score political points etc....


In the end its a complicated situation that will vary. Plus don't forget just because you have authority to request aid doesn't mean someone else hasn't requested earlier that you be denied aid; or that aid be restricted to you etc.... There are also chains of command even within the Inquisition; a lower ranking Inquisitor requesting high level aid without the backing of higher level Inquisitors might well not be refused, but it might be questioned and those being requested might well contact higher level Inquisitors to confirm etc...

 Overread wrote:
But the point is its still a request, the Marines can turn them down. It's basically a huge political dance. On paper they can do certain things, but the reality of the situation is that the paper is only as strong as the politics behind it and with the Emperor in limbo the powers behind rules and laws he put into place are in the hands of others.

And yes at times someone will demand something to the letter of the law and others will follow; perhaps just because they see value in the end goal and aren't looking to score political points etc....


In the end its a complicated situation that will vary. Plus don't forget just because you have authority to request aid doesn't mean someone else hasn't requested earlier that you be denied aid; or that aid be restricted to you etc.... There are also chains of command even within the Inquisition; a lower ranking Inquisitor requesting high level aid without the backing of higher level Inquisitors might well not be refused, but it might be questioned and those being requested might well contact higher level Inquisitors to confirm etc...

Which is pretty muchy what I said in the previous post but the fact remains that Marine Chapters are subject to the High Lords, the Inquisiton and the Primarch all of whom could order them destroyed and have done so in the past.

Marine Chapters have even been ordered to suicide run into the Eye of Terror and similar as pennance.

Request or order, in the end it is much the same, just a matter of degrees of politeness. Whether or not any organization can refuse will depend on its political capital and allies.

In theory, Chapters are completely independent, arguably not even technically a part of the Imperium, and so in theory a Chapter could say no to any and all requests for aid or demands/orders by others. However if it consistently does so and lacks the powerful allies to back it up, it may find itself like the Celestial Lions and start having higher than expected casualties or supplies from the Adeptus Mechanicus might get delayed and so on. It might also end up like the Astral Claws, accused of private empire building and ultimately the target of actual armed reprisals from a variety of other Imperial factions. It's politics.

Like anyone else with that level of power - a Chapter Master (Or Arch Magos, Planetary Governor, Fleet Admiral, Canonness etc ) would have to have a good reason to refuse aid - and usually why would they? If they have concerns about the Inquistors "request" then the really powerful ones may well be able to contact another Inquisitor ally who they have aided in the past - another reason NOT to refuse them without a really good reason is that they may be useful in the future.

Wrong, they can just say "Nope" and leave it at that. In fact the Dark Angels are massively guilty of this and why the inquisition has often been on the verge of branding them as heretics.

The breaking up of legions would have been extremely stupid if instead of a space marine you just have an inquisitor able to command them. The truth is that the Astartes chapters are under no obiligation do do anything an inquisitor tells or asks them to do. While it is smart to play nice with others they do not have to and its that simple.

This Founding involved the division of the existing Loyalist Space Marine Legions into the far smaller, 1,000-man autonomous Chapters of the Space Marines as decreed by the new Codex Astartes, written by the Ultramarines' primarch, Roboute Guilliman.

BrianDavion wrote:

and in each of thsoe examples while they have considerable influence they cannot COMMAND a space marine chapter. when the high lords want a space marine chapter to do something they REQUEST it. that applies to gulliman as well

Well, the Inquisition disagrees, they think their authority is limitless, the Astartes think they can treat them as suggestions. They've had wars over this.


Automatically Appended Next Post:

Rebel4ever85 wrote:
Wrong, they can just say "Nope" and leave it at that. In fact the Dark Angels are massively guilty of this and why the inquisition has often been on the verge of branding them as heretics.

The only reason the Dark Angels can say "Nope" is they're a powerful chapter who would exact a terrible price on anyone trying to enforce an Inquisitorial order on them.

Totto wrote:
Sure this has been debated before, but can't find any posts when searching.

I mean, I know they can take command of Imperial Navy ships and regiments (but how much? can they say "i take command of this superbig fleet or all Astra Militarum forces in this sector")

Can they go to Macragge and order the Ultramarines around? "like hey i take command of this company or even Chapter, I want you to do this and that"

Can Hector Rex or someone like him order the Custodes around?

De Jure, lots. De Jure, they can go to Maccragge and order the Ultramarines around. By the books, they have entirely unchecked and unilateral power to do everything except govern [though they can remove governing officials and appoint their own, and exercise emergency powers to some degree]

De Facto, their power extends exactly as far as they can make it. If an Inquisitor went to Fenris and told Logan Grimnar what to do, and Logan Grimnar said to shove off, it's unlikely the Inquisitor would have any recourse [which has in fact happened]. Any first founding chapter has a lot of political clout, and it would be political suicide for the Inquisitor to challenge that, especially because their peers in the Inquisition would immediately jump on them.
'

It's notable that the ability to tell the Inquisition to shove it up their ass does not extend to space marine chapters that aren't first founding, except for some of the most reknown. If you're, lets say, the Lamenters, and you refuse to carry out the Inquisition's orders, they can brand you a heretic and you will find few friends to back you up on that.

The difference?

An Inquisitor can requisition Imperial Guard units. As they can the Imperial Navy.

An Inquisitor can request the aid of other Imperial armed forces. Because those other forces are largely autonomous.

So you could have excellent relations with a given Chapter. But their resources are finite, and they’ll have their own actions in mind. If they simply don’t have the forces to spare, there’s precious little the Inquisitor can do about it.

And, in a sense, the Inquisition’s fractious nature is its own power check. Yes, you could declare exterminatus - but you’d better have good reason, because other Inquisitors will be keeping their eye on you and your actions, just as you do them. It might very well the solution in the end, but you need to be able to defend your actions. One does not squander His Assets lightly.

Consider the Kryptman Gambit. That was done with the implications considered and accepted. The mitigation is that all the time the Orks and Nids are knocking seven bells out of each other, The Imperium is bought time to better organise its forces for the inevitable fallout.

The power an inquisitor has is related to the forces at their disposal. They can conscript units, sure, but so can other imperial factions. I would say 'how big is the problem' and 'who is responsible' are other big factors. But whoever wins the fight, will dictate back to the imperium what happened (and yes, this will be highly biased in their own favour).

Rebel4ever85 wrote:

Like anyone else with that level of power - a Chapter Master (Or Arch Magos, Planetary Governor, Fleet Admiral, Canonness etc ) would have to have a good reason to refuse aid - and usually why would they? If they have concerns about the Inquistors "request" then the really powerful ones may well be able to contact another Inquisitor ally who they have aided in the past - another reason NOT to refuse them without a really good reason is that they may be useful in the future.

Wrong, they can just say "Nope" and leave it at that. In fact the Dark Angels are massively guilty of this and why the inquisition has often been on the verge of branding them as heretics.

The breaking up of legions would have been extremely stupid if instead of a space marine you just have an inquisitor able to command them. The truth is that the Astartes chapters are under no obiligation do do anything an inquisitor tells or asks them to do. While it is smart to play nice with others they do not have to and its that simple.

This Founding involved the division of the existing Loyalist Space Marine Legions into the far smaller, 1,000-man autonomous Chapters of the Space Marines as decreed by the new Codex Astartes, written by the Ultramarines' primarch, Roboute Guilliman.

Sadly you are very mistaken - A Inquisitor has the right to order Marines to aid him or her as per the actual GW lore. Now enforcing that is another story so powerful first Founding Chapters can and have said no but they have also said yes. Inquisitors have the right to do pretty much anything but have to be able to justify it to their peers.

The idea that the Marines answer to no-one is nonsense - RG is now the supreme commander of the Imperium military and orders the Chapters to do things as needed - as do the High Lords, as they have always done. Remember that a whole bunch of Chapters were Ordered to suicide run into the Eye of Terror as pennance and did so.

If the Astartes do not answer to anyone you end up with the The Tyrant of Badab which is why the Inquisiton (see a theme here?) keep an eye on the marines and how big and powerful they are getting.

The Space Wolves in particular have been acting more and more like the Tyrant - having vast non astartes military and refusing the gene-seed tithe/inspection. The Dark Angels are little better and the Ultramarines have their own mini Empire but First Founding Chapters get away with alot because of who they are and who they have allies with.

Both sides usually play nice - the Inquisitor "asks" powerful factions for help and if they don't want to the Astartes (and others) are likely to cite reasons why they can't. Obviously this depends on the request. If a Inquisitor asks for say... a Squad of Marines to do something she is hardly likely to be refused but a order to mobolise the entire Chapter is going to be met with resistance.

Thinking about it, it may be all even more grey when it comes to astartes and the inquisition.

As stated before, out of respect, I believe they tend to request aid, though can demand it - which can cause other issues.

However, depending on the manor in which an astartes chapter is called to war by an inquisitor, the inquisitor would have little to no influence in how the astartes conducted themselves in that war. Reference, the Dark Angels actions in the War of Vraks... They were requested for aid, they came in, chose their own targets, executed them and left, to the ultimate detriment of the siege armies already deployed. I do not think an inquisitor requested the DA's aid at this point, though the token is the same. A marine chapter in a war theatre can almost do as they please and leave as they please also.

That may be the more truly autonomous aspect of chapters, in the sense they can act as they please in a war zone when either requested or ordered and not much can be done about it (though, most are professionals so will do the job they're required to do so). There's plenty of nuances to this also, if there is more than one chapter, especially a senior chapter etc... Marines tend to fall into a chain of command of other marine chapters out of respect and peerage, but in the case of one chapter, bought to a war zone by an inquisitor, they most likely have carte blanche in how they decide to act in the war zone with little influence from the inquisitor unless the chapter wants to play nice that is and be the inquisitors personal elite force or listen to and agree with the inquisitors suggestions for their role, but again, they may choose to prosecute their own objectives instead.

Inquisitors theoretically have the power to command the entire Imperium as they claim only to serve (and by extension are accountable to) the Emperor Himself.
In practice, it varies and it's almost down to celebrity status. An Inquisitor such as Coteaz or Karamazov can command Guard regiments, Sororitas convents, and Navy squadrons. Space Marine chapters are usually requisitioned via honour pacts or debts incurred by said Inquisitors.
A low ranking Inquisitor will usually command a group of handpicked individuals, see the Inquisitor game for an idea of how that works.
An interaction would probably go something like this:
Inquisitor "Governor give me all your forces in the sector."
Sector Governor "Haha no, get out of here you rat."
Same Inquisitor "Captain I require your platoon."
Guard Captain "Yes sir, right away."

all command structures are made up, noone can order or command anyone else.

The consequences of not doing what you're told by someone who society has agreed to give lots of power to tends to have donwsides though.

Even the lowliest civilian can say no to an inquisitor. They'll probably get shot for it, but they can still refuse.

So the idea that inquisitors can't 'command' space marines is stupid.

A chapter master can't command space marines if their troops refuse to accept their authority.


The difference is simply the degree of consequence faced as a result of refusal. An inquistor commands marines to do something and those marines say no. The inquisitor might have to stop and think about whether executing them is a smart move if they don't have enough backing to protect themselves from the chapter's retaliation.

The imperium is built on the philosophy of the biggest stick - it's a nesting doll of biggest sticks around bigger sticks and so on. And then there's the maneuvering to convince others your stick is bigger (from the god emperor himself is often a great proxy stick).

In the context of the imperium, an inquisitor is contractually allowed to command anyone. However different organisations of different levels of power feel less compulsion to follow through than others.

Custodes vs Inquisitors though,

Am I right in thinking even the newest off the factory line Custode is beyond reproach for any demand made of them (not from their custodes superiors) by inquisitor or even the high lords?

I think one problem people keep hitting is trying to devise a linear hierarchy for the Imperium when one, by design, doesn't exist.

Individual groups have a generally linear hierarchy within themselves, but it gets more blurry between different groups of the Imperium. Hence why there's such variation and why one Inquisitor might demand and another might request and another would be refused even by the same rank of someone in another organisation.

There's a constant state of power struggles, politics, flux and general situational variation that results in a complex constant power struggle between the different parties. This is how its supposed to work, the idea being that no single group can ever gain majority power because others will pull them down or rise up above them. It's a constant state of flux to prevent a single group or person gaining majority power and taking over the Imperium. The only one who is allowed to do that is the Emperor who is, in effect, out of the picture at present.

I like to think of the Imperium as the Old West in the USA. There were laws created by the federal government and states, but a random town in the middle of no where out West was really under no obligation to enforce those laws.

Imagine an inquisitor is a US marshal tracking a group of five horse thieves across the Old West. The marshal finds out that the thieves are staying in a motel in a small town in the middle of no where. This individual marshal cannot apprehend all of the thieves at once by himself, so when he gets to town he goes to the sheriff and asks him and his deputies to assist him in the arrest. The sheriff says, "No." The marshal then orders the sheriff to aid him because the law of the United States gives him the power to commandeer local law enforcement to help him. The sheriff still says no because his brother is one of the horse thieves.

Technically, this marshal has the power to order around the sheriff, but the sheriff can still refuse because he knows that lone marshal cannot actually do anything to him to force him to follow the law. If he tried to arrest the sheriff for breaking the law the sheriff's deputies would stop that from happening. They may even kill the marshal to get him out of the way.

Now, if the same US marshal had rolled into town with 30 other marshals, and they all showed up at the sheriff's office the sheriff may be more inclined to lend aid even if his brother was one of the thieves. That isn't to say the sheriff would be obliged to do everything within his power to help. He might just feint support while secretly informing his brother the marshals are in town.

While it is true that Inquisitors answer to no one, and, by the letter of the law, can tell anyone in the Imperium what to do that doesn't mean that the people they are ordering around care about that law or hierarchy. Much of the authority in the Imperium is based on strength or power(real or perceived) rather than the letter of the law.

Gir Spirit Bane wrote:
Custodes vs Inquisitors though,

Am I right in thinking even the newest off the factory line Custode is beyond reproach for any demand made of them (not from their custodes superiors) by inquisitor or even the high lords?

If memory serves, only The Emperor may order a Custodes to do anything.

But, that does not in itself mean they won’t act on requests from others. Now I’m not all that up on my Custards lore, but I’d imagine they’d still need approval from their own chain of command. Nor can I really recall whether their Head Honcho has a seat on the High Lords, or if it’s just a really, really stupid idea not to pay heed.

What is clear (to me at least) is that the Adeptus Custodes can do pretty much as they please, without seeking consent or agreement of other parties. They’re not beholden to the High Lords as other departments are (even Astartes Chapters, who risk censure). And their involvement with Guilliman is at least by agreement - and possibly even the direct order of The Emperor himself.

One can imagine similar Inquisitorial "unstoppable force" meeting "immovable object" scenarios.

Like an Inquisitor attempting to revoke the Warrant of an ancient Rogue Trader family, with claims that the Charter was originally granted by the Emperor directly.
The Rogue Trader could claim the Inquisitor cannot undo something the Emperor did.
The Inquisitor could dispute the claim the Emperor actually directly gave the Warrant.

The Inquisitor could also appeal to the Emperor's authority by declaring "If by declaring this Warrant now null and void, I am going against Your Will, strike me down!"....then when silence and nothing happens, conclude that therefore the Emperor does not object.