Necron Allies

So I intend to play a 2v2 game this Friday with a bud of mine who is playing a lesser race, Necrons. Never had an issue with 2v2 allies as it's just fun to have a big game among friends. I always disliked the ally chart in the BRB though, because if Eldar were to ally with Dark Eldar, they would be a fool not to keep "one eye open."

But as I started forging the narrative in my head, I began to wonder if this is the only alliance that would "work" for Dark Eldar. I'm not incredibly knowledgeable about races outside of dark eldar aside from bits and pieces. I don't read the books. I'm still struggling to get through the path series. That's why I'm posting this here so someone can correct me if I'm wrong.

Necrons are soulless automatons, they offer nothing in terms of pleasure through torture.

They can phase out, even if you were able to capture them.

Tyranids are soulless too, I hear you say, but they offer entertainment in the arena at least, I can't see people lining up to watch a necron in hand to hand combat.

Yes, Eldar hate the necrons, they're an ancient foe, but putting aside a grudge to get something short term accomplished is natural for the Dark Eldar. I bet they can't even buy groceries without having to cover their resentment for the cashier.

On the flipside I think current fluff has necrons trying to empire build, and dark eldar could care less about that.

And if the fluff is still the same where necrons hate all living things, the Dark Eldar are basically already dead.

I guess what I'm proposing is that in this particular alliance, betraying your ally would serve no purpose other than just committing the act of betrayal, which admittedly would be enough for some Archons. Both parties would gain something by allying, and gain nothing from killing each other.

That depends on your narrative.  Is the Necron Lord trying to expand his empire into territory filled with the souls of lesser races?  Is there a Webway Gate nearby that would be threatened by the expansion of the Necrons?  Necron technology is very advanced, even by Dark Eldar standards, do they have some new weapon your Archon covets?

There is always a good reason to betray your allies!