In the troops tab of the database, you'll see the section called Battle Event. In Condition, you can choose when the event plays, and you can plan it out like a normal event underneath that. Read the tooltips to get a better grasp of what things do. The Span decides when the event will run; if it is set to Battle it will only happen once in the battle, if it is set to Turn it will only happen once per turn, if it is set to Moment it will repeat until the conditions are no longer met.