Clash is a keyword ability that appears on cards from Lorwyn and Morningtide. Clash provides an added effect to these spells and abilities if the revealed top card of your library has a converted mana cost that is higher than the revealed top card of your opponents’ library. After these cards are revealed, each player that revealed a card can choose to put the revealed card on the top or bottom of their library.

Here is a specific example of how it works. Player A plays Lash Out targeting a creature controlled by his opponent, player B. When Lash Put resolves this targeted creature will receive three points of damage and then both of these players will reveal the top cards of their libraries. If the converted mana cost of the card revealed by player A is higher than the converted mana cost of the card revealed by player B, then player B will receive three points of damage. If the converted mana cost of the card revealed by player A is the same or lower than the converted mana cost of the card revealed by player B, then no damage is dealt. Then both players will choose whether to keep these revealed card on top of their library or put it on the bottom.

There is one ‘wrinkle’ with clash in multiplayer games. When an effect instructs a player to ‘clash with an opponent’ the controller of the spell or ability with clash chooses any opponent in the game to clash with. This means that a player can play Lash Out and target the creature controlled by one player but choose to clash with another. All that matters is that the player that is chosen to clash with is an opponent. Also, making this choice does not target, so a player that cannot be targeted can be chosen to clash with.