At the far end of the hall, the face of a monstrous creature is carved into the stone wall of an alcove. Water trickles from its mouth into a large stone basin directly under it in the floor.
Anyone who walks down the center of the hall, instead of staying near one of the walls, activates a hidden pressure plate, firing eight barbed javelins from the monstrous creature’s mouth, four in the first round and four more in the second.
The pressure plate can be spotted with a DC 15 Wisdom (Perception) check or a DC 15 Intelligence (Investigation) check. The javelin-hurling slot in the stone face can be spotted with a DC 17 Intelligence (Investigation) check. Wedging a shield or other object in front of the mouth does not prevent the trap from activating, but it catches the javelins before they go far, and jams the trap.
Barbed Javelin Trap. Ranged Weapon Attack: +7 to hit, range 50 ft., up to four targets or a single target four times. Hit: 27 (6d10) piercing damage, or half damage with a successful DC 16 Dexterity saving throw.