Murderous Redcap

Persist turns a single enters-the-battlefield damage trigger into two, and that doubling is the entire engine. The body comes back once after it dies, but only if it had no -1/-1 counter, so it returns as a 1/1 and fires the damage again. That second trigger is what made this a fixture of sacrifice-loop construction: any outlet that lets you kill it on demand, paired with a way to strip the counter or grant it a fresh death, converts the card from a two-for-one into an arbitrarily repeatable ping. Because either half of the cost can be paid with black or red, it slots into mono-black, mono-red, or both at once, asking almost nothing of the manabase while paying back a removal spell stapled to a recurring threat. What balances it in isolation is the modesty of the numbers: the damage scales off power, and power here is small, so the unbroken version is a fair piece of attrition. The broken version is a combo piece, and that gap between the two readings keeps it resurfacing. Persist is the pessimist's recursion keyword, returning a creature weaker rather than stronger, and this is its cleanest argument: a removal spell that refuses to stay used, and that pays you twice for the privilege of finally putting it down.