Merieke Ri Berit

A theft engine and an assassination tool wearing the same 1/1 body, and the tension between those two jobs is the entire design. The control clause is conditional and fragile: you hold the stolen creature only "for as long as you control Merieke Ri Berit," and the death trigger fires the moment she leaves play or untaps. That second condition is the cruel part. Because she never untaps on her own, any effect that does untap her (your own, an opponent's, or a misplayed Twiddle) snaps the leash and kills the borrowed creature outright, no regeneration allowed. So she rewards a deck that keeps her tapped and protected, then treats the steal as a delayed removal spell you can cash in on your terms: bounce her, sacrifice her, or let her die in combat to drag the best creature on the board down with her.

What makes the design durable is that the worst case is still a good case. If your opponent removes her, the creature you borrowed dies anyway; if you cannot hold the loan, you trade her for their bomb. The early three-color identity (this predates the modern wedge-and-shard vocabulary by years) signaled who she was for: a grindy control or value shell that wants its best removal to also be its best threat. Few cards from her era still read as cleanly as a piece of interaction.