The rules for Controlling another player's turn were written specifically for one card, Mindslaver, although the cards Sorin Markov, Worst Fears, Emrakul, the Promised End, and Cruel Entertainment now make this ability available from other sources, and Oracle text for Word of Command has its controller control a player while it's resolving. Mindslaver was designed by Mark Rosewater. It was originally conceived as a card for Tempest called Helm of Volrath, but R&D didn't like the original card. It was then going to be put in Unglued 2, but that set was put in indefinite hiatus. It finally made it into Mirrodin with the blessing of Paul Barclay, the rules manager.
From the Comprehensive Rules (Aether Revolt (January 20, 2017))
- 714. Controlling Another Player
- 714.1. Some cards allow a player to control another player during that player’s next turn. This effect applies to the next turn that the affected player actually takes. The affected player is controlled during the entire turn; the effect doesn’t end until the beginning of the next turn.
- 714.1a Multiple player-controlling effects that affect the same player overwrite each other. The last one to be created is the one that works.
- 714.1b If a turn is skipped, any pending player-controlling effects wait until the player who would be affected actually takes a turn.
- 714.2. One card (Word of Command) allows a player to control another player for a limited duration.
- 714.3. Only control of the player changes. All objects are controlled by their normal controllers. A player who’s being controlled during his or her turn is still the active player.
- 714.4. If information about an object in the game would be visible to the player being controlled, it’s visible to both that player and the controller of the player. If information about cards outside the game would be visible to the player being controlled, it’s visible only to that player, not the controller of the player.
Example: The controller of a player can see that player’s hand and the face of any face-down creatures he or she controls.
- 714.5. While controlling another player, a player makes all choices and decisions the controlled player is allowed to make or is told to make by the rules or by any objects. This includes choices and decisions about what to play, and choices and decisions called for by spells and abilities.
Example: The controller of another player decides which spells that player casts and what those spells target, and makes any required decisions when those spells resolve.
Example: The controller of another player during his or her turn decides which of that player’s creatures attack, which player or planeswalker each one attacks, what the damage assignment order of the creatures that block them is (if any of the attacking creatures are blocked by multiple creatures), and how those attacking creatures assign their combat damage.
- 714.5a The controller of another player can use only that player’s resources (cards, mana, and so on) to pay costs for that player.
Example: If the controller of a player decides that the controlled player will cast a spell with an additional cost of discarding cards, the cards are discarded from the controlled player’s hand.
- 714.5b The controller of another player can’t make choices or decisions for that player that aren’t called for by the rules or by any objects. The controller also can’t make any choices or decisions for the player that would be called for by the tournament rules.
Example: The player who’s being controlled still chooses whether he or she leaves to visit the restroom, trades a card to someone else, agrees to an intentional draw, or calls a judge about an error or infraction.
- 714.6. The controller of another player can’t make that player concede. A player may concede the game at any time, even if he or she is controlled by another player. See rule 104.3a.
- 714.7. The effect that gives control of a player to another player may restrict the actions the controlled player is allowed to take or specify actions that the controlled player must take.
- 714.8. A player who controls another player also continues to make his or her own choices and decisions.
- 714.9. A player may gain control of himself or herself. That player will make his or her own decisions and choices as normal.
- You can see everything that player can see but you normally could not. This includes that player's hand, face-down creatures, and any cards in his or her library that he or she looks at.
- You control the entire turn, from the untap step to the cleanup step.
- You could gain control of your own turn using Mindslaver, but gaining control of your own turn doesn't really do anything.
- You don't control any of the other player's permanents, spells, or abilities.
- You can't make the other player concede. A player can choose to concede at any time.
- You get to make every decision the other player would have made during that turn. You can't make any illegal decisions or illegal choices -- you can't do anything that player couldn't do. You can spend mana in the player's mana pool only on that player's spells and abilities. The mana in your mana pool can be spent only on your spells and abilities.
- You choose which spells the other player plays, and make all decisions as those spells are played and when they resolve. For example, you choose the target for that player's Shock, and what card that player gets with Diabolic Tutor.
- You choose which activated abilities the other player plays, and make all decisions as those abilities are played and when they resolve. For example, you can have your opponent sacrifice his or her creatures to his or her Nantuko Husk or have your opponent's Timberwatch Elf give your blocking creature +X/+X.
- You make all decisions for the other player's triggered abilities, including what they target and any decisions made when they resolve.
- You choose which creatures attack and how those attacking creatures assign their combat damage.
- You also make choices for your own permanents, spells, and abilities as usual.
- You can't make any decisions that aren't called for or allowed by the game rules, or by any cards, permanents, spells, abilities, and so on.
- If you make another player play Shahrazad, you don't control any of that player's turns in the subgame, but you continue to control the current turn once the subgame is completed.