The Gloomer was an enormous, frightening, mindless black water rat that lived in the underground lake of Kotir and was controlled by Tsarmina Greeneyes and her army. Verdauga captured and imprisoned him long ago. He was used as a sort of attack animal to fight the wildcat's enemies. Upon Tsarmina's request, Brogg, Scratt, and Cludd fastened some leads on the Gloomer and Fortunata led Brogg, Scratt, and the beast into Mossflower to fight. The Gloomer got into the waters of the River Moss and met his demise upon battling the vicious Stormfin, an enormous pike, in the River Moss.
The Gloomer was blind and hard of hearing, but had enhanced smell. He did not exhibit self-consciousness and was veritably insane from many seasons of imprisonment in the dark lake.