Staff of Power
Okay, there is a planet, and on this planet there is a staff, and this staff has the power to return one person to life and in full health. Every leader of every army is sent a nice little postcard telling them about the staff and about it's powers. Every army then decides "Hey, we could bring our super-duper fluff buffed character back from the dead!" Then every race sends a massive army to the planet to claim their prize. However, every army reaches the planet at the exact same time.
Since Tyranids couldn't read the postcard they had a massive army there just by chance, and instead of wanting the staff they want the genes of some super-organism native to that planet.
All of the armies want the staff/super-organism, so they will not use any weapons or tactics that would have the potential to misfire and destroy the planet.
Which army would win out and claim their prize?