Shoggoth Missile

Shoggoth Missile is a dreaded - and mostly forbidden - relic from the age of Silinrul Imperium. A missile filled with bioengineered plague that can wipe clean entire worlds in few days, devouring and assimilating all biological matter. Even worlds that manage to survive the outbreak are in most cases permanently scarred, most of their ecosystem dead and big parts of planets scorched by nuclear fire used to push the plague back.
Shoggoth Missiles are rarely found on a wrecks of the biggest of the Imperium's warships spit out by the anomalies or found in the depths of space. They are actually banned by the Outer Colonies - one of the few actually common rules the place has - and overwhelming majority of countries buys them only to properly dispose of them, rather than letting them fall into the hands of radicals.
It is caused by the fact that the Shoggoth Plague wiped out most sapient species in the southern part of the Milky Way Galaxy during the late stages of the Darkening. If it mutated once, it can probably mutate again so it's a better idea to avoid the risk altogether. At least unti Mankind reaches the level of Ancients that managed to stop the last outbreak.