The Arcane Realm is the name given to the pocket dimension created by the magician Gwydion. Unsatisfied with reality, Gwydion sought to fashion a world to his own liking. He used his abilities to create a realm outside of time and space, where a spirit could leave their body and travel to a fantastical land. Because it is subject to the whims and wills of the spirits who occupy it, the Arcane Realm is fundamentally unstable and highly dangerous.
Creating the Realm directly resulted in Gwydion's death, but his spirit became trapped in the Realm, unable to move on even after his body perished. Over time the spirits of other mortals who sought power and agency beyond what reality could offer them became ensnared by the Realm, which entrapped their spirits upon their deaths, populating this world with the souls of the damned.
Role of Tarot
Tarot cards, a popular tool for divination, are believed to have been devised by Gwydion as a method of introducing others to the Realm. A cult has sprung up around the cards among those who believe in their power to tell the future, and adherents to this belief often see the Major Arcana as helpful or even divine spirits who want to help people.
Since it has no definite shape or form, the Realm can take on any appearance. Some have described it as looking like a paradise, while others say it is a hellscape or a wasteland. Whether it is the stuff of dreams or nightmares depends on the person, why they are there, who they associate with, etc.
It appears that Gwydion has more control over the Realm than others. He is most commonly found on an island covered in dense wildlife and surrounded by a sparkling, crystalline sea. The weather on the island is shown to be influenced by his moods.