Overlord of the Balemurk

Every entry and every attack digs four cards deeper into your own library and hands back the best non-Avatar creature or planeswalker from your graveyard. That makes the 5/5 both the shovel and the thing you dig up: it builds the graveyard it then mines, self-feeding without ever needing another card to prime the pump. The non-Avatar exclusion is the leash that pays for the whole engine. Because it cannot return copies of itself, the obvious infinite is foreclosed and the returns stay pointed at other threats you actually wanted buried. Impending is where the flexibility lives. Cast at full cost, it enters as a body that immediately mills and recurs; cast for its cheaper Impending price, it arrives as a noncreature enchantment carrying five time counters, ticking down one per your end step until the last one comes off and it wakes up as a creature. The key is that the mill-and-return trigger cares only about the permanent entering, not about it being a creature yet, so the cheap line buys the value on turn two and defers the 5/5 for later. Play it early as a grinding enchantment that does nothing but stock and refill your hand, or drop it late as a finished attacker: the reward is identical, and the only decision is when you feel like paying for the body.