Demonlord of Ashmouth

The entry tax and the death-payout pull in opposite directions, and that contradiction is what makes the card more than a flier with a body. The exile-unless-you-sacrifice clause demands a creature on the way in, a real cost for a 5/4 flier at four mana; undying then guarantees the body comes back the first time it dies. Stack those two against each other and the math gets interesting. Each return through undying triggers the enter-the-battlefield clause again, so the demon keeps asking for a sacrifice every time it cycles back, which is exactly what an aristocrats engine wants to be fed: a recurring sacrifice prompt attached to a body that refuses to stay dead. The counter undying leaves behind is also the catch. Once it carries a +1/+1 counter, undying stops working, so the recursion is a one-time guarantee unless something strips the counter or doubles its dying. The card sits at the seam between two black instincts that rarely share a frame: the sacrifice-as-cost school that prices creatures by what you feed them, and the resilient-threat school that wants a flier you cannot answer with a single removal spell. Most designs pick one. This one bolts them together and lets the entry tax become a feature rather than a drawback, because the creature you sacrifice to it is often a creature you wanted in the graveyard anyway.