Cavern Harpy

The pay-1-life return clause is the engine, not a panic button: it lets the enter trigger fire again for a life apiece, and what it returns is any creature in its own colors that you control. Mate that loop to a useful enters-the-battlefield trigger and you have a repeatable machine costing only life and recast mana. Ravenous Rats becomes a discard faucet; Man-o'-War becomes recurring bounce; any drain or mill body in the right colors becomes a tap you can run dry. The card itself never insists on a win, which is what makes it dangerous: it is a free component that hands the rest of the combo to whatever creatures it shares colors with. The 2/1 flier and the mandatory return-a-creature trigger are the costs that pay for the loop. It cannot resolve its entry without sending something home, so a board with no second eligible creature forces it to bounce itself; the fragile body earns its keep as a piece rather than a threat. As a design, it is one of the cleanest cases of a creature built to be a verb instead of a card, the kind of self-recurring enabler that combo decks would lean on long after it left constructed play.