Kezef The Chaos Hound
Kezef, the Chaos Hound, is a monstrous skeletal dog with maggots in place of flesh and black acid blood. Maybe one of the two unmentioned The Seven Lost Gods of Antiquity. He exclusively consumes the souls of the faithful in the afterlife (those who focus their worship on one god), a permanent way for GM to kill a character even with a super wish. Who may have been created by Jergal. As the gods got annoyed with being unable to claim their best followers, they chased it to Barrens of Doom and Despair (a plane that exists in the World Tree and World Axis model), like the story of Fenrir, the gods challenged Kezef to break out of their chains made by Gond. Confidently agreeing, but only if he gets to bite someone's arm off if it does work, Tyr performs the same role as the Norse version and sacrifices his arm. Kezef was chained, and Mystra created a magic self-repairing veil to double insurance, with no deity or mortal being allowed to interact with him, Being Kept fed with some divine essence from the arm.
During the Time of Troubles, reinforcing the question of why other gods tolerate the Dread three, Cyric, using Mask as his sword, released Kezef out of pettiness in the hope he will eat Kelemvor. The fact that Kelemvor is now listed under the deity table shows how much this plan didn't work. Chasing Kelemvor to Waterdeep, with the help of Mask and Adon, temporarily trapped him in a candle before thinking to release a Elder Evil to track down Cyric was a good idea.
He is now on the run from the god and Has a grudge against Mask, which is a good thing because this drives it to spend more time laying low and hunting down Mask than eating souls.
In third edition, Kezef is CR 21.
5e's Sword Coast Adventure guide has Kezef as a possible Warlock patron but only mentions the story about how he was imprisoned for the first time (and not that he escaped a long time ago-- fail? or retcon?).
