Íllgrathr appear most of the time as small, thick, ivory-colored worms which move around by slingshotting themselves forward on minuscule hooks which cover their whole body. When they find something to eat, however, it is revealed that these three-inch worms are actually multiple tiny green centipedes which appear to split from a singular grub. These centipedes burrow into their victims’ bodies and eat their flesh from the inside, usually killing their prey in the process. When their meal is finished, the centipedes reform back into the ivory-colored grub and they move on to find their next meal.