When you set this scheme in motion, target opponent chooses a player. Until your next turn, only you and the chosen player can cast spells and attack with creatures.
"I am nothing if not fair. I've chosen my champion. Now you must choose yours."
The player is chosen as the ability resolves. Once a player is chosen, it's too late for other players to respond by casting spells.
15-06-2010
wotc
The targeted opponent may choose you. In that case, only you can cast spells and attack with creatures before your next turn begins.
15-06-2010
wotc
The effect doesn't wear off until just before your next untap step (even if an effect will cause that untap step to be skipped).
15-06-2010
wotc
Players other than you and the chosen player may still perform game actions besides casting spells and attacking with creatures: They may block with creatures, activate abilities, perform special actions, and so on.
15-06-2010
wotc
The targeted opponent may choose themselves.
Random a card of Magic: The Gathering (MTG), but you can also roll the D20 dice, random a Planeswalker, have fun with the random rules and more.