This lore can be found by an Inquisitor on Voss.
Little is forbidden on Voss, where life is directed by the infallible visions of the Mystics. Yet one ritual, practiced by ancient spirit healers and believed to be powerful enough to restore even the most fractured mind, was locked away within the Shrine of Healing and guarded by a Voss apparition. The so-called “Forbidden Ritual”–its true name long lost to history–involves dream-walking alongside one of the hated Gormaks. By uniting both Voss and Gormak, it can heal the mind of its “nightmares.”