Since ancient times the intersection below the Holy Sycamore of the Gwyd Mountain had been considered as a seat of unnatural power, thought by Aen Seidhe elves to be an incarnation of Mori Rígain. When Nordlings reached the area, they adapted elven beliefs and built a castle on the mountain around the sacred tree. Apart from the castle's rulers it became a seat of priestesses of Morrigan and the centre of her cult. Many religious writings were produced here and the sanctuary became a destination of pilgrimages.
The situation changed around the late 1250s, when Countess Ailen of Gwydberg banished the last of priestesses, Vela, because the latter accused Ailen's husband Constans of raping her. Unconsciously, Vela cursed the castle and the Sycamore heard her plead. In a disastrous storm the fortress was severely damaged and ravens started to attack the inhabitants forcing them to flee. The sole residents of the former sanctuary became were priestess Vela and cursed Ailen's daughter, Iola of Gwydberg.
Gwydberg remained a ruin until early 1270s when it was reached by two expeditions, one led by Kala of Gwydberg, Queen Cirilla's vassal, and another by merchant Bruno Messmer. Members of the latter discovered that the Sycamore was in fact a prison of an otherwordly entity trapped here millennia ago.
Not much is known about the elven practices devoted to the deity. When Nordlings adapted Morrigan to their pantheon, they built her a chapel on the Gwyd Mountain. At least one virgin priestess always lived in the castle, where she was taking care of the sacred tree, performed rituals for the goddess and tutored novices as well as the count's children. The cult spread to different regions but Gwydberg remained its very core.
- Morrigan is mentioned in The World of King Arthur. The Malady by Andrzej Sapkowski but only as a goddess in British Isles, without any suggestion of being worshipped on the Witcher Continent. In context of the latter she is described only in The Glass Mountain scenario, Copernicus Corporation's official Polish material for The Witcher Role-Playing Game.