Gwydion is the true embodiment of the Major Arcana. He can take on the form of any Arcana, although he was at one point mistakenly believed to be just the Magician.
In life, he was the eldest son of King Peredur. Unsatisfied with the real world, he used his powers to create the Arcane Realm, hoping to make himself a god over it. Made out of his magic, the Realm lacked the inner logic of reality but was influenced by Gwydion's own will and desires. Creating the Realm sapped Gwydion's strength completely, killing him. Upon his death, his spirit was unable to move on, remaining imprisoned within the Realm.
Lonely and bored in the cage of his own making, Gwydion managed to infiltrate the real world and set in motion his plan to create a new religion revolving around himself. He is implied to have created the tarot deck, which he uses to communicate with the living. In this way, he makes dealings with mortals, granting their wishes in exchange for something in return. Usually, he asks for their soul, but does not reveal what he intends to do with it.