Choose Your Champion | Magic: The Gathering
Formats Choose Your Champion is Legal in
Official Oracle Text for Choose Your Champion
Official Rulings for Choose Your Champion
6/15/2010 : 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.
6/15/2010 : The targeted opponent may choose himself or herself.
6/15/2010 : The targeted opponent may choose you. In that case, only you can cast spells and attack with creatures before your next turn begins.
6/15/2010 : 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.