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My mate and I are going ot be playing against Eldar and BA soon. We want to make up a background story for Tyranids and Necrons. Can you think of any situation where Necrons and Tyranids may join forces? Thanks
Tyranids don't like the taste of living metal. They much prefer the biomass that are flesh-and-blood armies. Necrons take advantage of this fact and go after Eldar and Blood Angels as well simply due to the fact that 2-vs-2 is better than 1-vs-3 (necron vs all three).
Record: Win - Loss - Draw: Hive Fleet Pandora (New) 32-6-6 Space Wolf 7th Co. 52-11-6
Blood Angels 12-4-2 Daemonhunters 20-8-3
Imperial Guards 12-5-2 Daemons 8-3-2
That's exactly what he gave you.
Knowing that the Nids aren't interested in attacked them, the necrons see the Nids are fighting the Eldar and Blood Angels and join in. You could even say that the Necrons trick/lure the Nids into attacking them before joining in to make the battle easier.
Oroto ego sum
Simple logic on behalf of both forces. Tyranids cant consume necrons due to phasing out, Tyranids dont have souls for for the necrons to consume (ive got that right havnt i? Necrons eat souls?), failing that necrons and eldar have been at it for yonks, theyd always kill the pointy ears first, before anyone else. So BA and Eldar unite against a common cause, but necrons and tyranids have nothing to gain by fighing each other.
However as has been noted, JY2 did give you a perfectly good explaination.
Your friendly neighbourhood gargantuan creature
You could always go with the speculation that the Tyranids were made by The Outsider. If you accept this then the reasons for the Tyranids and Necrons joining up are easy to explain.
Not a theory I'm a fan of though.
Oroto ego sum
In fact, in the fluff, nids go around necron tomb worlds. This is the only case where nids will not absorb a planet's biomass. You can build any number of reasons just based on that fact.