Flamewake Phoenix

The recursion clause is the whole engine, and it is built to be cheap on purpose: pay one red mana at the beginning of combat, and a dead phoenix walks straight back into the red zone, no summoning sickness because haste comes stapled on. What makes that loop dangerous rather than fair is the condition gating it. Ferocious asks only that you control one creature with power 4 or greater, a threshold red aggro decks clear without bending their curve, so the recurrence cost is a single mana paid over and over while the board pressure that enables it is already part of the gameplan. The forced-attack clause is the price: this is not a blocker you keep back, it commits to combat every turn it can, which is exactly what an aggressive deck wants from a two-power flier and exactly what makes it a liability anywhere else. This is the recursive red threat sharpened: rather than a flat "return it when it dies" bribe, the resurrection is tied to the same big-creature board state aggressive red is trying to assemble anyway, so the trigger fires precisely when the deck is doing what it wants. Burn it, block it, chump it, and as long as a fat threat survives, the phoenix is one red mana from the air again next combat. The design rewards going wide and tall at once, and it taxes the opponent's removal at a rate most decks cannot sustain.