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Dagoth Uthol is one of the Ash Vampires and brother to Dagoth Ur. Vivec suggests killing him in order to weaken Dagoth Ur.
If you kill Dagoth Ur first, all the other Ash Vampires will die with him. Dagoth Uthol may be found in Kogoruhn, Charma's Breath. He carries the Belt of Heartfire artifact.
Of the seven Ash Vampires, Uthol is unique in that he is the only one who will let you pass peacefully if you talk to him first. Just choose "come to submit" when he talks to you, and he'll let you go.
