Nether Spirit

The trick is in the upkeep clause: keep your graveyard clean and this Spirit comes back free, every turn, forever. That single condition (it must be the only creature card in your graveyard) turns an unremarkable 2/2 body into a recurring threat that demands an answer outside the usual creature-removal toolbox, since killing it just sets up its return. The deckbuilding cost is steep and deliberate: a list built around this can run no other creatures, or must scrupulously exile the ones it does, lest a second corpse in the bin switch the engine off. That restriction is what prices the recursion. The card pays for its near-immortality by demanding a graveyard it controls absolutely. Exile is the clean kill, but bounce, transform, and any effect that drops a second creature card into the yard all neutralize it just as well, which makes it a recursive threat that rewards opponents for thinking laterally rather than reaching for the removal pile. Black has revisited graveyard-loop bodies many times in the years since this kind of design first appeared, but few hand the answer back to the opponent so plainly: the off-switch is sitting in your own graveyard, and the whole game becomes a question of who controls what is buried there.