Clark Ashton Smith, "The Infernal Star", in Strange Shadows: The Uncollected Fiction and Essays of Clark Ashton Smith, Steve Behrends with Donald Sidney-Fryer and Rah Hoffman, ed., Westport, CT: Greenwood Press, Inc., 1989.
Mordiggian dwells within a temple in the city of Zul-Bha-Sair, and is served by a cult of ghoul priests.
Mordiggian turned up first in Clark Ashton Smith's short story "The Charnel God", which you can find in Necronomicon Press' _Zothique_ book.
Anybody else out there, correct me if I'm wrong, but I believe Mordiggian was originally created by our good ol' pal CAS, in one of his Zothique tales, I believe (BTW, looking at an old atlas th'other day I discovered a town in Quebec called St.-Zotique or something).