From Hearthstone: Heroes of Warcraft Wiki
Put a copy of a random minion from your opponent's deck into the battlefield.
Sometimes it feels like this is all a game.
 How to get
- This spell copies a random minion from your opponent's undrawn deck, not their total deck, and will not copy cards which have already been drawn.
- Unless it has Charge, the copied minion will be exhausted, and unable to take action that turn. Minions with Charge will be able to attack immediately. Any Battlecries will not be triggered, since the minion is not being played from the hand.
- If you play Mindgames and your opponent has no minions left in their deck, you will summon a Shadow of Nothing to the field.
- Mindgames can be used to grant you some strategic knowledge of your opponent. For example, any minion copied by Mindgames is currently in your opponent's deck, waiting to be drawn. Outside of arena, this means that if they have already played one of this minion, or if the minion is a legendary, you can be sure that they do not have one of that minion in their hand, while if they have only a few cards remaining, you can be certain that that minion will be drawn soon. If the Shadow of Nothing is summoned, this lets you know that they have no more minions to draw, which can be tactically critical. However, remember they may still have minions left in their hand.
- For 4 mana, this card will most likely copy a minion with a lower cost, unless the opponent's deck has a flat mana curve.