Event cards are played before each battle. There are typically 5 or 6 different events per mission set. You may use as many from each set as you see fit in your deck. Once played, they affect the current battle, or the overall game itself. They may have a minor impact on the upcoming battle such as “All Tactic cards with a Fighting icon are +1 this battle”. Or, they may have a much larger impact on the game such as, “One Front Line character of your choice is immediately KO’d”.
Below are some pictures of 2 sets (Infestation Incident & Into the Depths)