A type of Special Attack that indicates some effect will impact those inside a specific area of game play. From the Accelerant core rules book:
"Someone calls out "In This Room" and an effect and everyone in that room will be affected by it. Doorways and Gates act as room dividers. This delivery is commonly used for Traps and performances. The attack only works in an enclosed room with normal doorways and corridors leaving it. The attack will affect you if any part of you is within the room when the verbal is called."
The most common places to encounter this call are in cases where there are traps, but it is also possible for character abilities to have "in this room" effects (helpful or harmful). All traditional defenses, immunities, etc apply to any stated effects that happen after "In This Room..." verbals.