Reaction or Free Action

Most hazards have reactions that occur when they're triggered. For simple hazards, the reaction is the entirety of the hazard's effect. For complex hazards, the reaction might also cause the hazard to roll initiative, either starting a combat encounter or joining one already in progress, and the hazard continues to pose a threat over multiple rounds. Some hazards have a triggered free action instead of a reaction; for instance, an electric fence can shock multiple creatures per round.