Glimmervoid Basin

The triggered ability here is a fork engine welded into the tabletop: while this plane is out, every single-target instant or sorcery anyone casts keeps its original target and also splits into a copy for every other spell, permanent, card off the battlefield, or player it could legally target. A Doom Blade becomes a table-wide execution. A Giant Growth pumps every creature at once. Ancestral-style draw or a life-loss spell hits every player at once. The design turns the fundamental unit of spellcasting, the single target, into a broadcast, and it does so symmetrically, which is where the tension lives: the copies land on every legal target the caster can reach, but the plane doesn't care whose turn it is. That makes it one of the sharpest examples of the Planechase format's core idea, which is that the play mat itself becomes a rules object every deck at the table has to reckon with. The chaos ability doubles down on the copy theme from a different angle, seeding a chosen creature onto the board of everyone except its controller as tokens, so even the die roll pushes the game toward proliferation rather than removal. It rewards decks that were already leaning into cheap, flexible pointers and punishes anyone holding a single premium single-target answer, since that answer stops being surgical and becomes a shotgun. As a piece of multiplayer design it is less a card than a temporary constitutional amendment: for as long as it is the active plane, the whole group is playing a variant where nothing targets just once.