Sakashima's Will

The design conceit is a payoff for controlling a commander: control one as you cast, and you take both halves of the split instead of choosing one. The first mode is an opponent-chooses theft, a Threaten variant that keeps the creature permanently but hands the choice to the person losing it: they surrender their least valuable body, not their best, so read this half as attrition rather than a heist. The second mode is the sharper trick, folding every other creature you control into a copy of one you pick until end of turn. That is where the "both" clause earns itself. The copy mode is a static overwrite rather than a fresh arrival: creatures already on the battlefield do not re-enter when they become copies, so this is a combat and board-state move, not an enters-the-battlefield engine. What the copies bring along is the printed body plus the abilities of the chosen creature, which is where the leverage sits. The tether is that the both clause wants your commander already committed to the table, not lurking in the command zone, so the bonus scales with what you have actually cast rather than what your deck promises on paper. That contingency is the discipline: a flat four-mana blowout in any blue deck it is not, and it rewards the player who has put a commander at risk in the first place.