In addition to their standard Event text, all Event cards contain additional text (located at the bottom of the cards) representing their use as Combat cards.
Unlike playing an Event card for its main use, playing it as a Combat card does not require taking an action. Combat cards are played during a battle.
Just like with Event card effects, Combat card effects modify the normal rules of the game, and the text of the card always has precedence over the normal rules. Event cards used as Combat cards are always discarded immediately after use.