Witch's Mist

The condition is the whole design: it kills only what has already bled this turn. That makes it a cleanup spell rather than an opener, a way to finish a creature that traded into your blockers, walked into a ping, or survived a burn spell with a point left. The repeatable tap lets you lean on it across multiple turns, but each activation has to wait for combat or some other damage source to soften the target first, so the enchantment never operates alone. Pair it with anything that deals incidental, nonlethal damage at instant speed (a recurring pinger, a fight effect, a damage-dealing planeswalker) and you have a removal engine that delegates the hard part: the damage sets up the kill, the enchantment cashes it in. That division of labor keeps the rate honest, because three mana plus a tap for repeatable creature destruction would be wildly underpriced without the prerequisite, and it relegates the card to build-around status rather than maindeck answer. The clause cares only that damage was dealt, not that it was lethal, so the trick is finding sources that wound without finishing the job: a deathtouch blocker is no help, since that creature is already gone to state-based actions before you ever get priority to activate. The destroy clause is also useless against an indestructible creature, no matter how much damage you stack on it. Slow, parasitic, and dependent on a board state you have to manufacture, it belongs to decks already in the business of dabbing damage everywhere.