Keeper Bel'dugur is a level 42 Forsaken quest giver located in the Apothecarium
[53.7, 54.4]in the Forsaken's Undercity. From dialogue surrounding one of his quests, it seems he is a member of the Twilight's Hammer.
He starts the following quests:
[26D] The Book of Ur
[44+] The Star, the Hand and the Heart (Horde)
[40] Forbidden Knowledge (Alliance source: Gerrig Bonegrip)