Leech Collector // Bloodletting
A 2/2 warlock that turns black's incidental lifegain into a delayed drain, and the mechanical hook is the gap between arming and firing. The body sits on the board waiting for you to gain life for the first time in a turn, at which point it arms and offers a copy of Bloodletting to take two off each opponent. The trigger cares about the first lifegain in any turn, not just yours, so an instant-speed lifegain source can arm the warlock during a rival's combat; but Bloodletting is a sorcery, and the permission it grants is a continuous "you may cast," so nothing lands on the stack until you have priority at sorcery speed. You bank the armed state and discharge it during your own turn. Bloodletting is not a drain (it takes life without returning any to you), which keeps the engine from feeding itself: the lifegain that arms the warlock and the spell it produces draw from separate resources, so the loop is a metronome rather than a self-sustaining burst. Firing the copy strips the armed state, so each cycle demands a fresh trigger. Keep the creature alive, keep feeding it lifegain one point at a time, and it grinds the whole table down two life per turn.
