Greetings! I feel that the Mabinogion is an excellent example of mystical journey, the First Branch about the Prince of Annwfyn in particular. Pryderi agreeing to change places with the Prince of Annwfyn for a year and taking care of the kingdom and treating the queen with honor.

In a way the story of Gawain and the Green Knight, I feel, can be considered a mystical journey with the otherworld coming to Camelot in the form of the Green Knight. Then Gawain going to the kingdom of the Green Knight to fulfill his ageement. The fantastic events, I feel, being the events happening on this mystical journey.