Pandemonium

The first enchantment to turn a creature entering the battlefield into a damage trigger, and the engine that launched a whole family of combo decks built around it. The design move is subtle but radical: it detaches a creature's damage output from combat entirely, so a body that would normally have to attack, survive removal, and connect over a turn cycle instead points its power at a face the instant it enters. That collapses a creature's strategic clock from several turns to zero. The "may" clause and the controller-chooses framing keep it symmetric on paper, but in practice the player building around it is the only one prepared to exploit it: pair it with any way to flicker, reanimate, or repeatedly generate large creatures and the trigger fires again and again, each iteration a burn spell that scales with power rather than mana. The reason it became a combo staple rather than a Limited curiosity is that it rewards going wide and going big simultaneously, and it does not care whether the creature is summoning-sick, blocked, or stripped of abilities the moment after it arrives. Saproling tokens, freshly reanimated fatties, anything that hits the battlefield becomes ammunition. It reads like a fight enabler and plays like a kill condition you assemble around the creatures you were already going to cast.