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Chapter 10: Game Mastering / Hazards / Triggering a Hazard

Reaction or Free Action

Source Core Rulebook pg. 520 2.0
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 may 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, quicksand can suck down multiple creatures per round.