Chaos

Chaos is one of the most devastating spatial manipulation-branch weaponry, that is also known for causing particularly painful aftermath even to those that survived being hit by it. It is classified as worldkiller mostly because it is bad in hitting fast moving targets (like fleets), but truth to be told, if there was no enough power supplied to it, it could theoretically destroy STAR SECTORS, annihilating hundreds of star systems at once.
Chaos seems to be a variant of Reality Merger, whose effects - while require more energy - are much more permanent. Chaos, once fired, causes permanent merge between Reality and Hyperspace, causing the latter to permanently bleed into the Real Space, disturbing laws of physics. Nothing close to ground zero can possible survive the sideeffects of the merge, but the the real trouble occurs later.
The breech spreads. Slowly, but surely. Even if only a single planet in a system got destroyed by a Chaos, the subsequent wound in Reality will inevitably devour entire system. And will keep spreading... though the sheer size of universe prevents it from devouring entire star sectors.
Chaos is volatile to any human (and every other sapient species, though dragons seem very resilient towards its effects) and twists everything it devours. Ships emerge looking weird and being omnicidally agressive while phased out of Reality similar to Ghost Ships. Science fails, as it stops making sense (there was a case of space in three light years diameter around the wound developing Earth-like atmosphere and even LIFE, though the less is said about it the better). Reality is slowly but surely murdered into a state that was compared by someone to primordial chaos of Greek mythology (hence the name).
While Outer Colonies failed to outlaw the Chaos - mostly due to the fact that many countries wanted to keep experimenting on it, if only to find a way to 'fix' the already destroyed places - it stopped being used as weapon. But it still remains one, and if everything else fails, countries might resort to it once again.