Tivash, Gloom Summoner

The lifegain-to-tokens pipeline runs on a currency most black decks already print in bulk. The end-step trigger converts a turn's lifegain into a single flying Demon by spending that same life back, so the card is doing double duty: it wants you gaining life, and then it wants you spending it, which turns your life total into a battlefield resource rather than a safety margin. The lifelink on its own 4/4 body seeds the smallest version of the loop (attack, gain four, pay four, build a 4/4 flyer), but the ceiling is set by whatever drain and lifegain engine you stack around it, since X scales with the whole turn's total rather than any single source. What keeps it from spiraling for free is the payment clause: the Demon costs exactly the life you gained, so you never bank the gain and the token both, and a turn where you gained thirty and want a 30/30 leaves you back at your starting total. That symmetry is the design honesty at the center of the card. It rewards building toward big lifegain spikes without letting lifegain function as pure defense; every point that goes into the demon is a point that leaves your face. The result is a black finisher that punishes go-wide lifegain shells for hoarding life instead of weaponizing it.