Barrowin of Clan Undurr

Reanimation usually costs something: a discard, a life payment, a sorcery-speed window you have to survive to. This 3/3 turns it into a recurring attack trigger, and the price is paid up front rather than each time. The enter trigger takes the first step into a dungeon; the attack trigger, once you've finished one, hands back a small creature from your graveyard every time you swing. Front-loading the setup onto the dungeon crawl means the recursion asks nothing once the run is complete: clear a short dungeon early, then rebuild a board one three-drop at a time. The mana-value-three-or-less clause is the fence around the loop: no fatties, only the reusable bodies that keep an aristocrats or sacrifice shell fed. The body is deliberately unimposing, because the card is not a threat you protect but a spigot you attack with; losing it costs you the engine, not the game. It belongs to a small class of white-black creatures that treat the graveyard as a renewable resource without demanding tempo in trade, and the venture framing is the clever wrinkle: the dungeon is a one-time cost, and every attack after completion is pure return.