From Creatures Wikia
However, personality-wise, they are opposite to the ettins. While ashura ettins like brightly lit areas, the grendels prefer dark areas. They will attempt to go down with the lift soon after hatching, and every time they are in a brightly lit area.
What light will do to an asura grendel includes making him homesick, making him fall asleep, and killing him. However, the area has to be really bright in order for an asura grendel to die.
They can eat detritus and manky. Also, they feed off of norns by hitting them.
Besides attacking norns when hungry, the asura grendels are a peaceful race and rarely attack norns otherwise.