Mirrorweave

Commander is where Mirrorweave does its real work — the four-player table means more creatures in play, more targets, and a much higher ceiling on both the combo kill with Biovisionary and the chaos-mode political plays. In Legacy and Vintage it's legal but functionally a curiosity; fair creature strategies have faster, more consistent angles, and control decks don't want a four-mana sorcery that requires a board state to do anything. Modern is the same story — the four-mana slot is crowded with cards that win games more cleanly. Oathbreaker is a reasonable home if your signature spell setup enables token floods, but the smaller format makes assembling the necessary board width harder on average.