bone trap

you set an invisible trap that only triggers when the a NPC or player passes by. You can place the trap on any floor, wall, or ceiling. When triggered this trap sprouts 4 foot bones the trap will cover the whole surface of the place its set on.(if set the trap is set on the ground in the open the trap will sprout bones in a 100ft cube any NPC's that are smaller than 4 feet are grappled by the bones. all NPC's that are in the trap take 4d6 damage.