Avalon, also known as Otherworld, is a fictional dimension appearing in American comic books published by Marvel Comics. It is based on the mythical Avalon from Celtic and, more specifically, Irish and Welsh mythology.

Avalon first appeared in Fantastic Four #54 (September 1966) by Stan Lee and Jack Kirby. Avalon's dimension was referred to as 'Otherworld' for the first time in Captain Britain #1 (October 1976) by Chris Claremont, Herb Trimpe, and Fred Kida.

Avalon is a small other-dimensional planetary body located in a pocket dimension adjacent to Earth. It is formed by the collective subconscious of the inhabitants of the United Kingdom and the Republic of Ireland. The dimension of Avalon is also known as Otherworld. However, the island of Avalon within this dimension is actually only a small part of a larger dimension which is the Otherworld. Other places within this dimension include Fomor (home of the Fomorians), Annwn (home of the dead), and other locales associated with Celtic mythology. Notably, this pocket dimension also contains the Starlight Citadel, home of the fictional otherworld being, Roma, and the base of the Captain Britain Corps.

Otherworld is a high fantasy world; gods, elves, dragons, and many other beings live in small towns and large castles with no heavy industry. Large forests and bright lakes separate the several islands and locations from one another. Sorcery and magic are a part of daily life, although the Captain Britain Corps and Avalon also possess highly advanced technology.

Within the fictional historical setting of Avalon, there previously existed many ways to travel from Earth to Avalon and back during ancient times, which were created by the Twelve Walkers. Six of the Twelve Walkers created false paths that would lead travelers to dangerous places or drive them to the point of insanity, which resulted in a backlash from the human inhabitants of Earth against all magical creatures. At this point, many of the magical creatures relocated to Avalon and left most of the paths between Earth and Avalon destroyed, with only a few remaining functional.^{[citation needed]}

After the death of King Arthur, his body was taken to Avalon by Merlin. The city of Camelot was also taken to Avalon including all of the Knights of the Round Table, the process through which it was taken to Avalon was unclear.

Merlyn created the Starlight Citadel, located on a different island than Avalon. From the Starlight Citadel he founded the Captain Britain Corps, a group of superhuman heroes who would patrol the countless Earths in the Omniverse. Merlyn became the Omniversal Guardian and was assisted in his task by the Omniversal Majestor/Majestrix. His task was to safeguard the Omniverse against any threats and the technology Merlyn provided was capable of destroying entire universes that posed a serious danger. When Mad Jim Jaspers appeared, Merlyn faked his death and was replaced by Roma as Omniversal Guardian. Saturnyne became the Omniversal Majestrix.^{[citation needed]}

Years later, Mastermind conquered the island of the Starlight Citadel, destroying most of the Captain Britain Corps and disguising himself as Roma. Mastermind wanted the Amulet of Right and the Sword of Might, two mystical artifacts which had once empowered Brian Braddock, also known as Captain Britain and his enemy Joshua Stragg, the Reaver. Braddock found the artifacts before Mastermind and discovered that he was the rightful heir to them. Through the use of those artifacts, he was able to defeat Mastermind, and Roma named him the new ruler of Avalon, whilst remaining as his advisor. Brian recently left his throne after the events in House of M and returned to Earth.^{[citation needed]}