Going through the new Apocalypse rules and GW's take on what armies ally with others it says that just about any army can ally another given enough fluff to back it up. My friend, who really got me into 40k plays Necrons and Nids, I play Tau, and by GW, they dont have a shot in hell of allying up
. SO i take it upon myself to make up some shallow fluff to justify it
Necron/ Tau alliance
After the Harvest of Harvest of Ka'Mais the Tau were unhappy with the Necrons but although they considered themselves enemies, the Tau gained respect for them. The Necrons were quite and obedient of their leaders, they all fought together without question for their own sort of greater good. Now the Tau don't have a good enough understanding of the Necrons to know that they are all soulless robots bent on total destruction, rather another determined enemy. So they view a similarity between them.
The Tau, who continuing to expand now encounter more and more resistance ,without an Ethereal to guide them directly, so they look for an ally to help them; the Necrons. They speak with a servant of the Deceiver who, learning from the best, fills the Tau with false ideas that the Necrons are infact similiar to the Tau. They strike a deal to fight along side each other, the Necrons to help purge the galaxy and allow the Tau to spread the Greater Good. The tau are content, while the Necrons use them to kill their enemies and sacrifice their souls to the C'tan as well.
Planned on a 2 v 1 (1500+1500 V 3000) or 2 v 2 (3k V 3K) match.
-If the Necron/Tau side wins then they will continue on to fight each other as the necrons decide its a good time to eat souls
And the Tau/ Tyranid Allianc
much easier to go about, and very much similar to how GW explains Tyranids teaming with IG.
The Tau who follow their great Ethereal to lead them, are brought down a dark path. Allied with the ultimate predators, the Tyranids. The hive mind overruns the Ethereals little mental defenses and uses his body as an avatar to control the Tau.
-Tau will fight along side the Tyranids as long as an Ethereal is living.