Mire Blight

Pair this with any source of damage and the creature it sits on stops being a creature with toughness and becomes a creature that dies the instant anything touches it. That is the wrinkle that separates this from straight removal: it does not kill on its own, it rigs the next pinprick to be lethal. A single point from a one-drop pinger, a ping off a fight spell, a token chump-blocking, even the enchanted creature swinging into a 1/1 all become death sentences, because the "dealt damage" clause does not care about how much. The classic line is to stick it on something the opponent can no longer afford to attack or block with, then make them trade or hold it back; combined with any repeatable damage source it functions as a one-mana Pacifism that also folds the creature the moment combat happens. The cost is the same cost every Aura pays: two-for-one risk if the creature is bounced or sacrificed in response, and total dependence on a damage source to convert. It is the curse-style design that black returns to whenever it wants a cheap removal-adjacent effect that demands a little setup rather than handing you the kill outright.