Doomion is a demon encountered during the Underground Pass quest. When killed, it drops the amulet of doomion, which is needed to obtain Iban's shadow. Protect from Melee should be used during this part of the quest to save hitpoints for coping with its attacks.
It is possible to use the small bridges connecting the platforms as a safe spot to attack using Ranged, Magic, or with a halberd. In the quest, the player must also kill the other demons, Othainian and Holthion.