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I have been doing some serious pondering for a while on certain facts and speculations. Please, after reading this, tell me what you think would be the most likely outcome to each question.

Necron Facts:
Necrons are made out of a living metal.
The C'tan despise the warp and actively kill other civilizations that can access it.
The Cadian gate is designed to close off access to the warp.

Tyranid Facts:
Tyranids harvest all biomass such as organic matter, minerals, gases, etc, etc
Tyranids have the disturbing ability to shut off access to the warp in their general facility.
Tyranids have made no real pushes towards necron space, perfering to leave them alone with much more readily accessible sources of biomass nearby.

So, now onto some interesting possibilities.

A. If the Tyranids pushed into Necron space and began to fight the necron, what are the odds of the Tyranids being able to successfully devour the living necron metal? If this happened, would you say that the unlucky necron would effectively be permanently destroyed?

B. If the Tyranids and Necrons continue to avoid each other, basically like a "Hey, we don't really have any reason to kill each other, so do you guys want to take that half the galxy, and we take this half the galaxy?" agreement, what would you say the general outcome for the rest of the civilized galaxy would be.

C. If the galaxy became completely dominated by the Tyranids, and they attacked the Necrons, who decided to all defend one planet, while the Tyranids assualted with an entire galaxy's worth of biomass, who would win? Consider this based on two different possibilities. The Tyranids can or can not consume the Necron's living metal bodies.

D. Now, considering C, consider the outcome if the C'tan were busy with the awakening with the Void Dragon, and the traitorous Outsider.

And now to conclude my post: I spend way too much time thinking.

I think Tyranids can consume living metal. It just isn't very efficient and so far that's the reason why they don't normally target Necron held worlds.

 ENOZONE wrote:
I think Tyranids can consume living metal. It just isn't very efficient and so far that's the reason why they don't normally target Necron held worlds.

They don't target them at all, any times they come into contact with crons are by accident. Entire systems can be devoured, save for a single planet, if that that planet is a tomb world, and sometimes entire systems are bypassed simply to avoid waking any Necrons in it.

Due to how Necrons work, I highly doubt Tyranids can gain any biomass at all from them. Eating a Necron Warrior just means you have a Necron Warrior repairing itself in your stomach, ready to relentlessly claw its way back out.

Since Tyranids are purely organic, they have less defence against Gauss than other races, and Tesla will hurt them a lot more. They also have little in the way of complex strategy, meaning that even the most tactically lacking Overlord could reliably come up with counter strategies. Alongside a lack of general fear, the fact they readily come to you only makes the job of the Necrons easier. As well as Gauss, scarabs would be a major pain for larger organisms, since most of them rely on pure armour to avoid damage, and scarabs tend to like to eat armour. If anything, all it'd take is a large swarm of scarabs to remove the threat of an incoming Tyranid fleet, not by devouring the entire fleet, but by devouring the main ships. Just as deadly is the thought of Wraiths phasing inside them, or Flayed Ones appearing inside them, or even Deathmarks appearing inside.

The Necrons simply have so many counters to the Tyranids that, even if the Tyranids were to be victorious, the victory would have been exceedingly costly, and very likely Phyrric, and biomass gained to biomass lost ratios would be incredibly differing, with very little gained for what would undoubtably be a catastrophic loss.

 Avatar 720 wrote:

 ENOZONE wrote:
I think Tyranids can consume living metal. It just isn't very efficient and so far that's the reason why they don't normally target Necron held worlds.

They don't target them at all, any times they come into contact with crons are by accident. Entire systems can be devoured, save for a single planet, if that that planet is a tomb world, and sometimes entire systems are bypassed simply to avoid waking any Necrons in it.

Genuinely curious - do you have any references for this? I only know of one, I think in the 4th edition Tyranid codex, where they avoid one system, which was rumoured to be the Outsiders Dyson Sphere - not a planet, a whole system. My impression of this is Necron fans took that bit of fluff and ran with it, because I haven't come across anything stating Tyranids outright avoid Tomb Worlds.

 Avatar 720 wrote:
Due to how Necrons work, I highly doubt Tyranids can gain any biomass at all from them. Eating a Necron Warrior just means you have a Necron Warrior repairing itself in your stomach, ready to relentlessly claw its way back out.

Tyranids get the vast, vast majority of their biomass from the planet itself. Kind of like seeing an ant on a hamburger. You'd get as much from that ant as Tyranids get from the population of a planet, because the planet itself has so much more. The only reason the Tyranids attack defenders is defenders destroy key structures and life forms dedicated to eating the planet.

Even a barren, desert tomb world would hold a lot of value to the Tyranids. Silicates from the sand, liquids beneath the sand, the atmosphere. And not all tomb worlds are barren. So saying they wouldn't attack a tomb world because they won't get much from the Necrons themselves misses the point of why Tyranids attack worlds in the first place. They're not there for the population inhabiting the planet, they're there for the planet itself.

I'm not contesting that the Nids don't go for the planet and not the inhabitants, but I am saying that a Necron army present on the planet would force the hand of the Hive Fleet, and a lot of biomass would be used up in trying to remove the Necrons so they can eat in peace.

There's also the fact that an Overlord studying the Tyranid actions would realise what they came for, and could potentially order it destroyed, molecule by molecule, or even transmogrified into something lethal to the Tyranids. The Necrons aren't above bending the laws of physics and reality to their will.

The thing that got me curios about Necrons being eaten was merely the fact that after one was, say, dismembered and consumed by rippers, the rippers would then take the remains to the almighty digestion pool. The real question is if a necron could repair itself before the living metal is completely broken down and just adds to the resources of the swarm.

Necrons have no DNA to speak of, so they can't be consumed for any evolutionary purpose.

'Living Metal' is a misnomer, it means the metal acts organically while being metallic.

 MrScience wrote:
Necrons have no DNA to speak of, so they can't be consumed for any evolutionary purpose.

'Living Metal' is a misnomer, it means the metal acts organically while being metallic.

And Tyranids consume metals. Living metal would be no different if they managed to block a Necron from teleporting home and dropped it in a digestion pool.

Seriously though, I'm still interested in references of Tyranids avoiding all Tomb Worlds. Anyone have it?

Why would they consume metal? It has no biomass.

 MrScience wrote:
Why would they consume metal? It has no biomass.

Don't think of consume as them just eating metal. Its more complicated than that. The rippers can secrete juices that help them break down and absorb just about anything the hive mind needs. As for why they need metals and all that is the fact that everything revolves around absorbing certain elements and keeping them in certain ratios in order to keep stable life processes. You think about the fact that humans need iron and zinc and all those other trace elements, then imagine what a small hive ship would require. They go behind the main battles destroying and absorbing everything they need, which leads back to the whole necron issue. If you have a disabled necron getting digested by rippers and then added to the pools of biomass of the hive fleet, is the poor necron not dead? I believe I can recall stories from the necron codex of necrons sacrificing themselves by blowing themselves up to avoid capture or some such . Also, pretty sure that there are numerous reports of the main Necron dynasty leader killing millions, if not billions of necrons in order to restore order amongst the lesser dynasties under his dominion. So it appears they do not have some "Hey you destroyed every part of me except my fingernail, so no biggie, I can come back." clause.

 -Loki- wrote:
And Tyranids consume metals. Living metal would be no different if they managed to block a Necron from teleporting home and dropped it in a digestion pool.

Seriously though, I'm still interested in references of Tyranids avoiding all Tomb Worlds. Anyone have it?

It was in 3e Necrons, 4e Tyranids, 5e Tyranids, etc.

I can't recall if it is in 5e Necrons or the 6e rulebook.

5e Necrons states that some tombworlds were consumed by the nids. However, whether or not these were any tombworld or just tombworld which had ignorant life dwelling on their surface is (I'm pretty sure) not clarified. Frankly, I believe Nids would really only go for the tombworlds that have life present upon them. Attacking desolate, barren tombworlds is extremely inefficient as the biomass gained to the biomass lost ratio would be unfavorable to say the least with how Gauss weaponry works. Actually starts to make me wonder if Nids eat the doomsday ark, one power cell containment breach later and the entire world disappears...

 Void__Dragon wrote:

 -Loki- wrote:
And Tyranids consume metals. Living metal would be no different if they managed to block a Necron from teleporting home and dropped it in a digestion pool.

Seriously though, I'm still interested in references of Tyranids avoiding all Tomb Worlds. Anyone have it?

It was in 3e Necrons, 4e Tyranids, 5e Tyranids, etc.

I can't recall if it is in 5e Necrons or the 6e rulebook.

Technically, he is right on the part in the Tyranid Code, though it is up to speculation. 2 tentrils of a hive fleet almost ran into Solumnace, however, they made a major couse correction just outside of necron space and went dormant after unable to find easily consumable biomass sources.

I think people get too caught up on the "Bio" part of biomass. Tyranids consume everything that can be used to produce more tyranids and that includes a variety of other elements that can be found in other sources besides biological beings. Much like it is important for people to have a balanced diet, Tyranids need a balanced diet too and they get it by consuming other materials too. Biological beings are just the easiest place to get it. Tyranids are also designed to evolve, so they could evolve in a way to digest necron metal if they can't already. Also, in the current Necron codex, necrons can die if they sustain enough damage before repairing. Being broken down by very powerful acids seems like enough to kill a necron.

As for why Tyranids seem to avoid Necrons, aren't Tyranids attracted to psykers and Necrons have no psykers/presence in the warp. Tyranids are heading for Terra because they are attracted to the Astronomican. I think it's that Tyranids don't really see Necrons while looking for their next meal. So Tyranids only fight Necrons when they stumble upon them by accident.

 shamroll wrote:
I think people get too caught up on the "Bio" part of biomass. Tyranids consume everything that can be used to produce more tyranids and that includes a variety of other elements that can be found in other sources besides biological beings. Much like it is important for people to have a balanced diet, Tyranids need a balanced diet too and they get it by consuming other materials too. Biological beings are just the easiest place to get it. Tyranids are also designed to evolve, so they could evolve in a way to digest necron metal if they can't already. Also, in the current Necron codex, necrons can die if they sustain enough damage before repairing. Being broken down by very powerful acids seems like enough to kill a necron.

As for why Tyranids seem to avoid Necrons, aren't Tyranids attracted to psykers and Necrons have no psykers/presence in the warp. Tyranids are heading for Terra because they are attracted to the Astronomican. I think it's that Tyranids don't really see Necrons while looking for their next meal. So Tyranids only fight Necrons when they stumble upon them by accident.

I would argue the point that Tyranids arent really attracted to Psykers, I think its more along the lines that the Hive mind is on a completely different level of existance, and all the sudden it find these invisible lines that lead to a buffet that says, "Follow for food."

It's worth noting the Tyranids specifically lace metals into the shells and claws of their units. Carnifexen have adamantium laced tusks and claws, for example.

Consuming the living metal and then breaking it down could just lead to hardier Fex shells.

 ENOZONE wrote:
They also have little in the way of complex strategy, meaning that even the most tactically lacking Overlord could reliably come up with counter strategies.

Also; wait what? The Tyranids are noted repeatedly to demonstrate a devastating awareness of strategy. Calgar specifically noted that mistaking them for simple beasts would be foolish, having observed their tactics at work first hand.

ScreamPaste wrote:
It's worth noting the Tyranids specifically lace metals into the shells and claws of their units. Carnifexen have adamantium laced tusks and claws, for example.

Consuming the living metal and then breaking it down could just lead to hardier Fex shells.

True, but living metal is not a natural deposit or even naturally existing. From the necron fluff at least I don't see how tyranids would consume such a thing given that such a liquid metal is likely not able to be broken down by the normal digestion process explained in the tyranid fluff. Even if consumed it would likely be (cough*) pushed out, but ultimately not absorbed.

Even if consumed it would likely be (cough*) pushed out,

Impossible for most Tyranids, they lack an 'exit' to their digestive tract, being temporary war beasts they have no use for such things. :p

If it's true a Necron sufficiently damaged while repairing can be prevented from reforming or teleporting away I don't really see any reason the Tyranid 'process' wouldn't absorb the material for later use. I suppose we'll never know 100% whether it is possible to eat the stuff, but I don't think it's too far fetched. If it can be carried off world and stuck to a few MCs they'd likely find a use for it.

At the end of the day though, I doubt it would outperform the materials already in use anyway. They make titans and terminator armour with adamantium for a reason.

Fun to see some of the speculation here. And the Tyranids avoids the Crons like the plague because they can't win by devouring them, they will get a net loss. And yeah even a stupid or tarded Necron-Lord would come up with some strategy to defeat the Nids, while the Nids would need Screed to defeat the Crons in return.

I don't really see how the Nids should do it with just devouring live planets, I mean if a swarm of them came, Neptune would give them more nutrition than the Earth. That's science-fact from the Universe I think. There must be something about devouring organic material that fuels the metabolism of the Nids unexplained by the fluff.

 -Loki- wrote:

 Avatar 720 wrote:

 ENOZONE wrote:
I think Tyranids can consume living metal. It just isn't very efficient and so far that's the reason why they don't normally target Necron held worlds.

They don't target them at all, any times they come into contact with crons are by accident. Entire systems can be devoured, save for a single planet, if that that planet is a tomb world, and sometimes entire systems are bypassed simply to avoid waking any Necrons in it.

Genuinely curious - do you have any references for this? I only know of one, I think in the 4th edition Tyranid codex, where they avoid one system, which was rumoured to be the Outsiders Dyson Sphere - not a planet, a whole system. My impression of this is Necron fans took that bit of fluff and ran with it, because I haven't come across anything stating Tyranids outright avoid Tomb Worlds.

 Avatar 720 wrote:
Due to how Necrons work, I highly doubt Tyranids can gain any biomass at all from them. Eating a Necron Warrior just means you have a Necron Warrior repairing itself in your stomach, ready to relentlessly claw its way back out.

Tyranids get the vast, vast majority of their biomass from the planet itself. Kind of like seeing an ant on a hamburger. You'd get as much from that ant as Tyranids get from the population of a planet, because the planet itself has so much more. The only reason the Tyranids attack defenders is defenders destroy key structures and life forms dedicated to eating the planet.

Even a barren, desert tomb world would hold a lot of value to the Tyranids. Silicates from the sand, liquids beneath the sand, the atmosphere. And not all tomb worlds are barren. So saying they wouldn't attack a tomb world because they won't get much from the Necrons themselves misses the point of why Tyranids attack worlds in the first place. They're not there for the population inhabiting the planet, they're there for the planet itself.

True, but if you're fighting a force that can easily wreck your face and inflict a great deal of pain, it suddenly becomes less worth it.

ALso, Tyranids do rely on the warp. Many Tomb Worlds are soulless voids as far as the Tyranids are concerned (with one planet in the codex specifically having a generator that cuts the planet off from the warp completely). Tyranids don't want to get near that. Combined with the power of the Necrons and the fact that most Necorn Tomb Worlds are dead planets, Tyranids suddenly have very little reason to attack Necron planets. Keep in mind Tyranids only harvest surface Biomass, they don't bore into the crust and start tearing the crust up.

 McNinja wrote:

 -Loki- wrote:

 Avatar 720 wrote:

 ENOZONE wrote:
I think Tyranids can consume living metal. It just isn't very efficient and so far that's the reason why they don't normally target Necron held worlds.

They don't target them at all, any times they come into contact with crons are by accident. Entire systems can be devoured, save for a single planet, if that that planet is a tomb world, and sometimes entire systems are bypassed simply to avoid waking any Necrons in it.

Genuinely curious - do you have any references for this? I only know of one, I think in the 4th edition Tyranid codex, where they avoid one system, which was rumoured to be the Outsiders Dyson Sphere - not a planet, a whole system. My impression of this is Necron fans took that bit of fluff and ran with it, because I haven't come across anything stating Tyranids outright avoid Tomb Worlds.

 Avatar 720 wrote:
Due to how Necrons work, I highly doubt Tyranids can gain any biomass at all from them. Eating a Necron Warrior just means you have a Necron Warrior repairing itself in your stomach, ready to relentlessly claw its way back out.

Tyranids get the vast, vast majority of their biomass from the planet itself. Kind of like seeing an ant on a hamburger. You'd get as much from that ant as Tyranids get from the population of a planet, because the planet itself has so much more. The only reason the Tyranids attack defenders is defenders destroy key structures and life forms dedicated to eating the planet.

Even a barren, desert tomb world would hold a lot of value to the Tyranids. Silicates from the sand, liquids beneath the sand, the atmosphere. And not all tomb worlds are barren. So saying they wouldn't attack a tomb world because they won't get much from the Necrons themselves misses the point of why Tyranids attack worlds in the first place. They're not there for the population inhabiting the planet, they're there for the planet itself.

True, but if you're fighting a force that can easily wreck your face and inflict a great deal of pain, it suddenly becomes less worth it.

ALso, Tyranids do rely on the warp. Many Tomb Worlds are soulless voids as far as the Tyranids are concerned (with one planet in the codex specifically having a generator that cuts the planet off from the warp completely). Tyranids don't want to get near that. Combined with the power of the Necrons and the fact that most Necorn Tomb Worlds are dead planets, Tyranids suddenly have very little reason to attack Necron planets. Keep in mind Tyranids only harvest surface Biomass, they don't bore into the crust and start tearing the crust up.

Actually, the Tyranids could care less about the warp. Their own psyker powers are developed from the sheer gesalt will of the hive mind, and due to their unique signiture, they don't show up in the warp, instead creating a null void that warp energies cannot penetrate, as demonstrated when astropaths try to communicate with Tyranid war zones and fail to penetrate their warp shadow. Ironically, even the Tyranid fleet doesn't travel through the warp, instead using little ships called Narwhals that open large rifts for the fleet to pass through, though these rifts are heavily influenced by gravity fields, which makes hive fleets reach their destinations even years behind other ship as they have to travel from the edge of a system to reach the inner planets. While i do agree that having no warp signiture on a planet does protect it, if a fleet does go into the system to in their sequence of searching every gravity well in their path for planets to consume for biomass, there is no real reason to stop it from landing.

I think it's a little hard to talk about this given how shallowly necron technology has been discussed and how their fluff has gone through huge changes of late.

What we do understand is that the Necrons used to have close dealings with a group of entities that could be considered the even greater devourer. The c'tan have been portrayed in a number if ways in the history and one of them is as near limitless appetites with power to match. At other times they seems more like bored deities that have outgrown their nature and taken on personalities and projects. These days they seem like a fragment of such a bored deity but still an incredible and mysterious liability. Perhaps there is something that the aliens can sense in them that the imperium has yet to address. They are theist foreign elements in the mythos after all.

Beyond that I'm sure they are also just an incredible hassle. Can you imagine how good Necrons must be at killing living things in non game terms?

A few more points to consider (some have been touched on already):

-The majority of the damage done to the defenders is via airborne bio weapons that disseminate through the air and water, choking/crippling/killing the majority of life on the planet. Unfortunately those don't work very well against Necrons. Also, the attack has a heavy psychological element to it, which doesn't work well on the souless calculating Necrons.

-I think it could be possible to digest living metal, but the cost would be pretty significant. First, the majority of Necron forces (I'm thinking on the order of 90% or so) would phase out, then come back ready for more. Then the living metal would still be trying to repair itself, causing some serious heartburn.

-From what I understand, Tyranids don't do much 'mining'. Necron tombs are pretty deep underground, so I could even see a Necron Overlord just shut up the doors, and let the planet be consumed above. Only minor damage would happen to the Tomb. If the Necrons did fight back, the Tyranids would lose even more resources trying to invade a tomb (swarms would be funneled, the big bugs won't fit everywhere, and the tyranid "ships" couldn't bring any long range firepower to bear.

Beaviz81 wrote:Fun to see some of the speculation here. And the Tyranids avoids the Crons like the plague because they can't win by devouring them, they will get a net loss. And yeah even a stupid or tarded Necron-Lord would come up with some strategy to defeat the Nids, while the Nids would need Screed to defeat the Crons in return.

I don't really see how the Nids should do it with just devouring live planets, I mean if a swarm of them came, Neptune would give them more nutrition than the Earth. That's science-fact from the Universe I think. There must be something about devouring organic material that fuels the metabolism of the Nids unexplained by the fluff.

Why does everyone have that misconception? The Tyranids are not a horde devoid of all strategy. In fact, it's noted by Marneus Calgar that they're awfully good tacticians and strategists. You've also pointed out yourself why it wouldn't give the Tyranids a net loss to deal with them; the world they're on i worth a great big pile of nutrients.

Texx wrote:
-From what I understand, Tyranids don't do much 'mining'. Necron tombs are pretty deep underground, so I could even see a Necron Overlord just shut up the doors, and let the planet be consumed above. Only minor damage would happen to the Tomb. If the Necrons did fight back, the Tyranids would lose even more resources trying to invade a tomb (swarms would be funneled, the big bugs won't fit everywhere, and the tyranid "ships" couldn't bring any long range firepower to bear.

They actually strip the entire planet of soil and mine out any useful minerals, leaving a bare rock, they're be quite likely to come across a tomb, and they have tunnelers for assaulting such a thing.

Also their ships do have ranged firepower, they have specialist ships that straight up latch onto and crush or rip open other ships too, though.

C'tan stuff is all outdated fluff. They're no longer malevolent entities bent on destroying the Warp and harvesting souls. They're just shards/weapons used by disparate Necron overlords.

I know it sucks, but nothing we can do about it.

Didn't they used to be though, in the new fluff? And isn't there a chance some of them escaped? I wonder what's up with the void dragon/mars now.

 Texx wrote:
-From what I understand, Tyranids don't do much 'mining'. Necron tombs are pretty deep underground, so I could even see a Necron Overlord just shut up the doors, and let the planet be consumed above. Only minor damage would happen to the Tomb. If the Necrons did fight back, the Tyranids would lose even more resources trying to invade a tomb (swarms would be funneled, the big bugs won't fit everywhere, and the tyranid "ships" couldn't bring any long range firepower to bear.

They actually do a lot of damage to the planet itself with Magma Vents. Basically, they seed a planet with organisms that grow down into the planet, opening up huge fissures into the core of the planet to harvest gases and minerals. While this kind of thing isn't going to harm Necrons, the massive damage done to the upper layers of the planet likely wouldn't please the Necrons.


Automatically Appended Next Post:

 Lucre wrote:
I think it's a little hard to talk about this given how shallowly necron technology has been discussed and how their fluff has gone through huge changes of late.

The same problem exists with the Tyranid fluff. There's a lot of vague references to taking biomass, stripping a planet to bedrock, the shadow in the warp, non-psychic powers, gravity field mumbo jumbo and all kinds of things. None of it is explained in any detail. The fluff regarding them taking things like minerals from a planet was, IIRC, a White Dwarf fluff article. Not to mention Cruddace properly crapped on established 3rd and 4th edition fluff when he basically rewrote them for 5th edition (because I guess he thought their fluff wasn't popular for some reason?).

So Necrons and Tyranids are basically in the same boat when arguing fluff theories - both of them are barely explained, and they have received major fluff rewrites (Tyranids have actually had three, from Rogue Trader to 2nd edition, from 2nd edition to 3rd edition and from 4th edition to 5th edition, while Necrons have had two, from 2nd edition to 3rd edition, and from 3rd edition to 5th edition). So we've got barely explained fluff from various editions, White Dwarf articles and variations from good old chinese whispers.

So any fluff arguments concerning them - particularly both together, turns into a huge mishmash of differing information and a lot of speculation.

I did read the bit about shattering the c'tan again today. Apparently it involved hitting them with an un fathomable amount of energy. And then after that they went to sleep for some reason...

If the old ones thought the c'tan were bad ass and posed a complicated problem, but the Necrons just decide to turn cloak and shoot them all to death I'd say we haven't seen everything the Necrons are capable of. I mean the black stone fortresses are supposed to be slap dash anti c'tan measures right?

Tyranids avoid active Tomb Worlds, or Tomb Worlds that are also Dead Worlds. The old Necron Codex has some information about this, and the 5th ed Codex describes an inactive dynasty being destroyed by a Hive Fleet.

The reason they avoid active Tomb Worlds is because there is not likely to be anything on the planet except for the Necrons -- though mileage may vary nowadays since some Overlords may have living slaves on the surface world -- and trying to eat them wouldn't be worth the effort.

Inactive Tomb Worlds are more likely to have useful things on their surfaces, even if it's just water. Even if the 'Nids were to only suck up the water and move on without trying to crack the planet, the tectonic effects would be pretty devastating for the sleeping robots.

As for their consumption of living metal, I'm sure that they could break it down if it was inert. The effects would probably be similar to what happened when Ferrus Manus got his magic hands.

 -Loki- wrote:
Genuinely curious - do you have any references for this? I only know of one, I think in the 4th edition Tyranid codex, where they avoid one system, which was rumoured to be the Outsiders Dyson Sphere - not a planet, a whole system. My impression of this is Necron fans took that bit of fluff and ran with it, because I haven't come across anything stating Tyranids outright avoid Tomb Worlds.

In the Newcron Codex, there is talk of this. Something along the lines of Xenobiologists seeking an explanation for why the planet Solemnace remained untouched while all the other planets in the path of Hive Fleet Behemoth were destroyed.

That said, the Necrons clearly don't think that Hive Fleets are beneath their concern. The 'Blood Angels Alliance' incident (again, in the Newcron codex) indicates that the Necrons will go to some pretty extreme lengths to fight the nids.

The Necrons havee some very serious weaponry. The C'Tan are described as Godlike beings that have the power to 'extinguished suns and devour whole systems in black holes called into being by their reality warping powers' (loosely quoted). The Necrons were able to attack and split these beings, capturing their components in tesseract labyrinths and controlling them on the battlefield. "The Necrons focussed the unimaginable energies of the living universe into weapons too mighty for even the C'tan to endure."

I think - based on this - that the Necrons could probably defend against any Tyranid attack. I don't think they could take the offensive though, hive fleets are too diffuse and spread out. If there was a single 'Mega Brain' that could be removed to destroy the whole hive fleet, then the Necrons could probably take that out. Get orikan on board for some time-travel/forecasting action, use the Celestial Orrery... not much 'nids could do in response.