Many spells create ongoing effects that harm creatures that move through or occupy an area. The rules of this section apply whenever a spell or effect states that it creates a “Hazardous Zone.” Whenever a creature enters the effect’s area or starts its turn in the effect’s area, the hazardous effect triggers as described in the spell or effect’s text.
A creature can only trigger a given Hazardous Zone once per round, resetting at the end of the creature’s turn. This means that a creature that walks into a Hazardous Zone and ends its turn there will trigger the effect twice (once for entering, and once for starting its turn there). However, a creature that is pushed into the Hazardous Zone before its turn starts will only trigger the effect once (once for entering…the triggering limit doesn’t reset until the end of the creature’s turn).
Slow Forming
Where a spell or effect describes a Hazard Zone as “Slow Forming,” it means the effect is created slowly enough to allow creatures in the area a chance to escape before the area becomes hazardous. For the first round after the zone is created, creatures who start their turns within the Hazard Zone do not trigger the hazard. As soon as a creature leaves the Hazard Zone, they then trigger the hazard normally with any subsequent movement. So a creature that leaves the zone and then re-enters will trigger the effect when they re-enter the zone.