Reaper from the Abyss

The morbid clause turns a single death into a recurring removal engine, and the trigger is built to feed itself. Once the demon is online, blocking it or trading into it satisfies the very condition that powers its end-step kill, which means an opponent has to thread a turn where nothing on either side dies, an increasingly difficult ask as the board fills out. The non-Demon restriction is the load-bearing limit: it cannot point at itself, and in the rare mirror it cannot clear fellow Demons, so the destroy effect stays aimed outward rather than collapsing into a self-cleaning sweeper. Note the timing too: this is not a sacrifice you control but a destroy that resolves at every end step, yours and theirs, so an opponent's own combat or removal can pull the trigger on your turn and against their own board. As a six-mana 6/6 flier, it already closes games on raw stats; the morbid removal is the part that makes ignoring it untenable, because the body that wins races also strips away whatever could block it in the air. Black has a long shelf of fatties that demand an immediate answer, but where most of those threaten only damage, this one threatens attrition on a clock the controlling player rarely has to work to keep running.