Gravecrawler

The engine room of nearly every black sacrifice deck since it appeared, and the reason is a recursion clause that costs only one black mana, provided you control a Zombie. The clause is precise about where that Zombie lives: it must be on the battlefield, not in the graveyard, so Gravecrawler does not re-enable itself. If it is your only Zombie and it dies, it stays put. Pair it with a second Zombie that sticks around (a token maker, another tribal body, a sacrifice outlet that happens to be a Zombie) and the loop turns perpetual: cast, sacrifice, pay one, repeat. That dependency is what keeps the engine honest; it asks you to maintain a Zombie presence rather than handing you free recursion off any creature type. The "can't block" line is the other tax. This is a 2/1 built to attack and to die, never to anchor a board, and a recurring blocker for one mana would lock down combat in a way the design refuses to allow. The trade is the whole premise: a body whose death is not a loss but a reload, feeding anything that profits from creatures dying (Carrion Feeder, a Blood Artist drain, a generator that counts corpses) rather than from creatures surviving. It is the cleanest one-card statement of the aristocrats idea, gated behind a tribal condition mild enough that the rest of your Zombies satisfy it without effort.