Board Thread:Warhammer 40k General Discussion/@comment-7186329-20130329201743/@comment-3580035-20130426012557

I quote myself

As for (the Celestial Orrery's) potential use, you'll have a hard time getting the Necrons to cooperate with anybody. They don't have the patience for lesser species to mess around with it, and using it to just blow up Tyranids "'''upsets the fundamental forces of the galaxy, setting of a catastrophic chain reaction" (quote). It then takes "many thousands of years of constant and precise micromanagement" (another quote) to fix any premature detonations'''.

Quoted straight from the codex. You don't mess with physics. Its like when Orikan decided to go back in time to wipe out a Space marine company before it got to a battle. Whilst the plan worked, it altered the timeline; causing 3-4 chapters to participate in a massive retribution strike, which led to the Tomb World that destroyed the original company to be wiped out.