Stronghold Assassin

A repeatable sacrifice outlet welded to a creature kill spell, with the cost paid in bodies rather than mana. Tap it, feed it a creature, and a nonblack threat dies; it untaps next turn to do it again. That makes the assassin a soft lock against any board it can keep up with, as long as you keep finding fodder, which black has never struggled to supply (tokens, recursive bodies, the chump blockers you throw in front of attackers and then cash in for removal). The nonblack restriction is what keeps the engine honest: it cannot answer the mirror, and it cannot point at itself, so a deck built around it stays vulnerable to its own color. The other friction point is the 2/1 body, which makes the assassin a priority target the moment it lands. Then there is the timing subtlety that rewards a careful pilot: the sacrifice is a cost, not part of the resolving effect, so once you activate, killing the assassin in response no longer saves the target. The fodder is gone either way and the destruction still resolves, which means the right answer is to kill the assassin before it ever activates, not after. This is black control's recurring idea distilled into a single creature: turn attrition into a value engine rather than a one-time transaction, winning slow games by trading a resource you have in abundance for the one resource the opponent cannot afford to lose.