Galigothiel is (presumably) an entity invoked as part of spell, including "Begone, Blight! Begone, Pestilence! In the names of Galigothiel and Baelroth, I abjure thee begone!", an spell intended to drive goblins from village grain stores.
The spell was used by Doctor Strange to exorcise demonic entities tied to the Undying Ones, then, merging with Aradnea (who had been bred and mystically engineered to serve as a conduit for the Undying Ones), to banish all the Undying Ones possessing Atlanteans during the Fear.