End of Reason
Considered the most terrible and dreadful weapon of mass destruction known to Mankind, the so called End of Reason is... well, something that seems to be even more contrary to science that most supertechs already are. It is one of the rarest superweapons which, most likely, is for the best. End of Reason causes the target world to disappear. There is not a single clue as to where the place actually goes, but it certainly gone forever. That alone wouldn't make it so scary (though there is obviously a sort of a dreadful suspense in that), but the problem is that it causes vast and unpredictable changes even outside blast zone. This change include vast discrepancies in records and even memories pertaining to the 'disappeared' planet. Each time it did happen (can be count on fingers of one hand), there were at least five or six different and contradictory answers for questions about who inhabited the planet, why it was blasted into nothingness, who did it, and even when precisely it happened. Using End of Reason isn't explicitly forbidden as unlike Shoggoth Missile or Berserk Missile there is not even a theoretical risk of it causing the end of the world as we know it... but it's still so explicitly wrong that is practically stopped being used completely.