Kiki-Jiki, Mirror Breaker

The tap ability reads like a value engine and functions like a combo kill switch. Pair it with any nonlegendary creature whose enter-the-battlefield trigger or untap ability resets the activation, and the sacrifice clause stops mattering: you make a hasty copy, untap, make another, and loop until the board is wide enough to end the game that turn. Zealous Conscripts is the classic partner, untapping Kiki-Jiki on arrival so the engine feeds itself; Restoration Angel and Felidar Guardian do the same work by flickering the goblin back to untapped. That two-card pattern has been a fixture of combo decks for the better part of two decades, the red half of a deckbuilding axiom that says "haste-enabled copy plus self-untap equals infinite." The legendary clause on the ability is the real constraint, not the sacrifice: you cannot copy Kiki-Jiki itself, so the loop always requires an outside piece, which is why the goblin reads as a 2/2 that does nothing alone and an instant win the moment the right body lands. The haste on its own body matters too, since it lets you assemble and fire the combo the turn it resolves rather than telegraphing it across a full turn cycle. Few creatures have so cleanly split the difference between a fragile do-nothing and a one-card combo finisher waiting for its other half.