Plagued Rusalka

The repeatable -1/-1 here is fueled entirely by your own board, which makes this less a removal spell than a conversion engine: it turns dying creatures into shrink effects, one for one, at the cost of black mana plus a body each time. That economy points it straight at sacrifice decks, where creatures already want to die and an outlet that does something on the way out earns its slot even though every activation still asks for . As a piece of attrition it can grind through a swarm of small creatures or, with enough fodder, stack multiple activations on a single turn to kill something larger; the limiter is that the toughness reduction lasts only until end of turn, so it shrinks blockers and clears x/1s but cannot quietly mark a creature for death across turns the way a -1/-1 counter would. Its own stats barely register: this is a starting brick in the engine, not a clock. What keeps it from running away is the dual cost on every activation: each shrink wants both mana and a permanent, so you are spending resources to remove resources, and the math only favors you when your creatures were going to be sacrificed anyway. That narrowness is the point. This is built for a graveyard-and-tokens shell that wants to convert bodies into removal, not a generically efficient answer.