Necrotic Plague

A removal spell disguised as an Aura, with a clause that turns a single kill into a recurring infection. Most creature-targeting Auras leave the board the moment their host dies; this one returns from the graveyard attached to a creature an opponent controls, and because the enchanted creature is doomed to sacrifice itself on its controller's upkeep, the plague keeps hopping from victim to victim. You cast it from hand on whatever you most want dead, so the first kill is yours to aim. After that the dying player picks where it lands next, which means it cannot be steered into a single opponent's army at will, but it also cannot be ignored: left alone, it walks down a board one creature per turn cycle, a slow, self-resetting sweeper that asks the table to keep finding something to feed it. The interaction layer is where it gets genuinely strange. Creatures that cannot be sacrificed, or whose deaths trigger their own benefits, change the math entirely, and the forced-sacrifice clause overrides any indestructibility the host might carry. As black design, it sits at the intersection of edict-style sacrifice and persistent enchantment recursion, a chassis built to punish a board state rather than answer a single card. The cost is the patience it demands and the control it surrenders: after that opening shot, the chain belongs to whoever keeps losing creatures to it.