Biting Sphere of Baal Trap
This trap activates when a creature steps on a hidden pressure plate 10 feet away from the blank stone door at the end of the hallway, releasing a large, metal sphere from the door. The sphere rolls from the stone door all the way to the entrance in 1 round. A creature in the path of the sphere must make a DC 20 Dexterity saving throw, taking 22 (4d10) bludgeoning damage on a failed save, or half as much damage on a successful one. The DC is reduced by 1 for every 10 feet the sphere rolls before hitting a creature.
Prevention. A successful DC 20 Wisdom (Perception) check sees the slightly-raised floor tile that conceals the pressure plate beneath the webbing. If a creature burns the webbing in the area, it has advantage on this check. Wedging an iron spike or other object under the pressure plate prevents the trap from activating.