You control target player during that player's next turn. Exile Worst Fears. (You see all cards that player could see and make all decisions for the player.)
4/26/2014 The player you’re controlling is still the active player during that turn.
4/26/2014 While controlling another player, you also continue to make your own choices and decisions.
4/26/2014 While controlling another player, you make all choices and decisions that player is allowed to make or is told to make during that turn. This includes choices about what spells to cast or what abilities to activate, as well as any decisions called for by triggered abilities or for any other reason.
4/26/2014 You can’t make the affected player concede. That player may choose to concede at any time, even while you’re controlling them.
4/26/2014 You can’t make any illegal decisions or illegal choices—you can’t do anything that player couldn’t do. You can’t make choices or decisions for that player that aren’t called for by the game rules or by any cards, permanents, spells, abilities, and so on. If an effect causes another player to make decisions that the affected player would normally make (such as Master Warcraft does), that effect takes precedence. In other words, if the affected player wouldn’t make a decision, you wouldn’t make that decision on their behalf.
4/26/2014 You also can’t make any choices or decisions for the player that would be called for by the tournament rules (such as whether to take an intentional draw or whether to call a judge).
4/26/2014 You can use only the affected player’s resources (cards, mana, and so on) to pay costs for that player; you can’t use your own. Similarly, you can use the affected player’s resources only to pay that player’s costs; you can’t spend them on your costs.
4/26/2014 You only control the player. You don’t control any of their permanents, spells, or abilities.
4/26/2014 If the target player skips their next turn, you’ll control the next turn the affected player actually takes.
4/26/2014 Multiple player-controlling effects that affect the same player overwrite each other. The last one to be created is the one that works.
4/26/2014 You could gain control of yourself using Worst Fears, but unless you do so to overwrite someone else’s player-controlling effect, this doesn’t do anything.
7/13/2016 While controlling another player, you can see all cards in the game that player can see. This includes cards in that player’s hand, face-down cards that player controls, and any cards in that player’s library the player may look at.
7/13/2016 Controlling a player doesn’t allow you to look at that player’s sideboard. If an effect instructs that player to choose a card from outside the game, you can’t have that player choose any card.