Xantcha, Sleeper Agent

A creature you give away. The design gamble here is total inversion: you cast a three-mana 5/5 by handing it to an opponent, then spend the game turning their forced beatstick against them. The three clauses lock onto a single strategic axis. Xantcha attacks each combat if able, so whoever holds her can never sit back; she can't swing at her owner or their planeswalkers, so the aggression always points somewhere else at the table. And the drain-and-draw is an open ability, activatable by any player, which is the wrinkle that makes this a genuinely multiplayer engine rather than a two-player curiosity: the whole pod can chip her controller down two life at a time, and each activation rewards the player who pays for it with the card, so the incentive to keep pinging is distributed across everyone except the controller. The problem the card sets you is how to build around a body you don't control. The answer is to weaponize donation: hand her off, then punish the recipient with pings, forced combat, and the shared drain, treating this creature as a liability you've foisted on the person you like least. It sits in a small lineage of ownership-manipulation cards that treat control and ownership as separate resources, but few make the split the entire premise. A five-power body that would be a clean aggressive drop in any other shell instead becomes a problem you've politely asked someone else to solve, badly, for as long as she stays in play.