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So when building 'Nids one can take a detachment of Genestealer Cult as they share Tyranids Keyword.

The GS Cult allows a detachment of Astra Militarum.

SO could a 'Nid force have

1 Detachment of Tyranids
1 Detachment of GS Cult
and
1 Detachment of Astra Militarum

All in one force?

I would say yes. The last sentence under Brood Brothers would indicate that you could still choose Tyranids as your Faction, bringing in Genestealer Cults on the Tyranid keyword.

Yeah, I would say that you can, since the rule says to ignore the Astra Militartam units when picking what faction keyword is your common faction. So that means you could pick tyranid and everything would be kosher.

No you cannot. When making a army you must select a Faction and all the detachments must be from that faction.

You only get 1 detachment of AM for each detachment of GSC. To get a GSC detachment it has to be a GSC Faction army. Since Tyranids do not have the GSC keyword they cannot be included.

Wagguy80 wrote:
No you cannot. When making a army you must select a Faction and all the detachments must be from that faction.

You only get 1 detachment of AM for each detachment of GSC. To get a GSC detachment it has to be a GSC Faction army. Since Tyranids do not have the GSC keyword they cannot be included.

Are you literally searching for all these threads to give the wrong answer? Here, let me copypaste this for the 3rd time.

Hey guess what, not only are you totally wrong from a rules standpoint, the official FAQ's answer this. https://www.warhammer-community.com/2017/07/02/warhammer-40000-faq-now-available-july2gw-homepage-post-1/

Index Xenos 2 FAQ wrote:Q. Can I have a Battle-forged army that consists of one Detachment of Genestealer Cults, one Detachment of Astra Militarum (using the Brood Brothers rule) and one Detachment of Tyranids?
A. Yes.

You don't "choose" a faction when you're making your army, you choose a common faction keyword, which GSC allow you to ignore for AM units.

Yeah but is specifically states you can only take 1 AM detachment for every detachment in which EVERY model has the GSC keyword. So with the FAQ you can now take a 3rd detachment with Tyranids and GSC mixed or all Tyranids whatever. But it requires 3 detachments to do so it depends on your groups limitations or event limitations.

Wagguy80 wrote:
So with the FAQ you can now take a 3rd detachment with Tyranids and GSC mixed or all Tyranids whatever. But it requires 3 detachments to do so it depends on your groups limitations or event limitations.

So, what you're saying is, it only works if you do it exactly how the OP originally suggested it would work? And how everyone else agreed it would work?

With the FAQ that I just read yes. The actual index specifically lists only Astra Militarum and Genestealer Cults can be in the same army. It said nothing about Tyranids.

Wagguy80 wrote:
With the FAQ that I just read yes. The actual index specifically lists only Astra Militarum and Genestealer Cults can be in the same army. It said nothing about Tyranids.

It doesn't have to; you are the one that made a false assertion about a detachment only being genestealer cult if it's part of a GSC army.
The Brood Brothers rule defines its own terms: an "Astra Militarum Detachment" is one where all the units share the AM faction keyword according to Index: Xenos 2 p.112. Similarly a "Genestealer Cult Detachment" is one where all the units have the GSC faction keyword, per the same page.

That means you can't have a tervigon and some cultists in the same detachment and call it a GSC detachment, even though that's still a legal Tyranids detachment.
It does not mean that units in a GSC Detachment lose their Tyranids faction keyword, or that a Detachment cannot be both GSC and Tyranids.

Further, the only requirement of a Matched Play army regarding Faction is that all units have one in common; it also does not say that the various Detachments can only include models with that faction or that they lose any other faction keywords they might have. Normally this would prevent you having AM and Tyranids in the same army, but the Brood Brothers rule allows you to simply ignore all units with the AM faction keyword when determining your army faction.

So you have:
Detachment A. Factions: Tyranids, <Hive>
Detachment B. Factions: Tyranids, Genstealer Cults
Detachment C. Factions: Astra Militarum, <Regiment >, maybe a couple of others (ignored for army faction purposes)
What do they have in common? Tyranids.

None of this was errata'd or changed by the FAQ.

That was due to the way the roster was laid out which says "Army Faction:_________" not Army Factions. So it was my assumption "Faction" being singular meant you had to choose GSC, Tyranids, Imperium, etc.

As for the other stuff your ranting about I dunno you've sprung a cog or something. I never said anything about anything losing any keywords.