Head Games

Target opponent puts the cards from their hand on top of their library. Search that player's library for that many cards. The player puts those cards into their hand, then shuffles.

Darkness

Prevent all combat damage that would be dealt this turn.

Xiahou Dun, the One-Eyed

Horsemanship (This creature can't be blocked except by creatures with horsemanship.)
Sacrifice Xiahou Dun, the One-Eyed: Return target black card from your graveyard to your hand. Activate only during your turn, before attackers are declared.

Choice of Damnations

Target opponent chooses a number. You may have that player lose that much life. If you don't, that player sacrifices all but that many permanents.

Koskun Falls

At the beginning of your upkeep, sacrifice Koskun Falls unless you tap an untapped creature you control.
Creatures can't attack you unless their controller pays {2} for each creature they control that's attacking you.

Season of the Witch

At the beginning of your upkeep, sacrifice Season of the Witch unless you pay 2 life.
At the beginning of the end step, destroy all untapped creatures that didn't attack this turn, except for creatures that couldn't attack.

Cruel Entertainment

Choose target player and another target player. The first player controls the second player during the second player's next turn, and the second player controls the first player during the first player's next turn.