Karontor, like his brother Grolantor, is one of the third generation of the Giant Pantheon, one of the so-called "runts." He is far more evil than his brother, and far more intelligent. Some myths say he was once beautiful and shining, but his jealousy of Stronmaus caused him to twist into a form as hideous as his heart. As the corruption grew, he descended into the underworld where he learned dark secrets from an ancient race of subterranean hags. On his return, he used this magic to twist some of the fairest mortal giants into fomorians and verbeegs as hideous and corrupt as he is.
Karontor dwells in a mausoleum of beast and giantish bones in Minethys, the third Layer of Carceri, which he calls the Rack of Injustice. He is served by petitioners and by a pack of ever-hungry winter wolves. He was banished to Carceri by Annam, who wearied of the endless fights between his sons and found Karontor the easiest to rid himself of.
Sources: Monster Mythology, On Hallowed Ground, Planes of Conflict, http://www.wizards.com/rpga/downloads/LG_Deities.zip