Dude, Nightbringer would slaughter them, thats what he does all the time. Its the very meaning of his existence. The Deciever would work, he could think up some nasty suprise for his various enemies. But on the other hand, Nids and Necrons aren't meant to fight side by side. Its like having Imperial Fists and Iron Warriors throwing a party. Just doesn't work fluffwise.
The C'tan exist on the souls of living beings and creatures, so there are two possibilities for what would happen. Theory A, the C'tan see all of the easily harvestable souls of the tyranids and chow down. Or B, the tyranid soilders have no souls, and the C'tan see no point in being near them.
I actually considered this, since I'm a necron player and am just starting tyranids.
I figured it would have something to do with the Deceiver taking over a Norn Queen. Assassinating the queen with Pariahs then setting up a beacon of some sort that would allow the Deceiver (or one of it's lieutenants) take the reigns and set the tyranids to chowing down on planets while the deceiver eats all the lost souls. (This would completely erase it's victims from existence which I found delightful.)
That's the best I've come up with, though I thought necrons might find killing ALL life in the galaxy counterproductive since then the C'tan would have to eat stars instead of souls.
Regardless of the fluff I come up with my friends generally refuse the idea of my controlling both armies at once.
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