- You may be looking for the alternate dimension with the same name.
Avalon was a planet which was native to a species which later became known as the Cephlie, who had formed a highly advanced civilisation which used nanobots to manipulate the world around them. After their technology backfired, the Cephlie returned to a simple life. The planet was colonised by humans in the 22nd century, who were forced into a medieval lifestyle in response to the nanobots.
Geography
Avalon was divided up into several nations, one of which was Elbyon. The capital of Elbyon was Glazebry, and the capital of the South Share (one of the four regions of the nation) was Fluxford. The settlements Thule and Borea were located in the north of the nation, and Silverwood and Westhold were two other locations in Elbyon that were not far away from one another.
Avalon had several rivers, including the River Dellberry and the River Flux. The River Flux fed into the Circle Sea, a sea 500 miles in diameter formed by the impact of a meteorite. The Sea contained a group of islands called the Shadow Isles, and bordered the south and east of Elbyon. An ocean bordered the northwest of Elbyon, and in the southwest there was a Crystal Forest. (PROSE: The Sorcerer's Apprentice)
History
Age of the Cephlie
The Cephlie were an advanced civilisation which ruled over and inhabited Avalon. They invented a system which was focused solar energy on large astronomical objects, such as the large array of dangerous asteroids present in the planetary system. This system included six artificial moons positions equidistantly around the planet, which simultaneously became a source of power for the planet. As well as this, the Cephlie invented highly advanced self-replicating nanobots, which were powered by the moons. Eventually the nanobot system became so highly sophisticated that it allowed the manipulation of energy, force, and matter via the mind. An immensely powerful master control was created to handle the most complex tasks.
These inventions proved to be the eventual downfall of the Cephlie, as the ability to do anything at will meant the loss of creativity and ingenuity. In around the 10th century, one Cephlie, Klist, grew tired of the arrangement of the stars in the night sky and so he tried to move them, only to find the effects of the nanobots did not reach that far. Instead he used to master control to send the planet's star supernova, leading to the end of his civilisation as the Cephlie switched to a simple life.
First colonists
In 2145, early human colonists arrived on the planet and began to settle. Their machines and technology were destroyed by nanobots, preventing their departure. Also, though not designed for their minds, the nanobot system began to detect the thoughts of humans, creating creatures from myth and legend as real, leading to the fantastical planet that would become known as Avalon. Such creatures included mermaids, dragons, and kraken. It even gave form to the gods of the colonists, leading to destructive battles between rival gods. In desperation, one colonist used the master control to wipe all knowledge of religion from the minds of the colonists, erasing the gods for good.
With modern technology not functioning because of the nanobots, the human colonists fell into a Medieval way of life, with the fantastical twist that accompanied the likes of dragons and wizards. The Kingdom of Elbyon was eventually formed, with cities such as Fluxford and Glazebry becoming major seats of power.
The end of magic
The First Doctor, Susan, Barbara, and Ian visited Avalon in 2991, where they met Bron of Westhold and became embroiled in a quest to defeat the powerful sorcerer Marton Dhal. Meanwhile, the crumbling Earth Empire had also become interested in the planet, having had several ships become untraceable in the system, and in desperate need of whatever power was capable of doing so. As such, Admiral Nyborg was dispatched to take the planet by force. With the three powers - King Magnus (with the Doctor on side), Marton Dhal, and the Earth Empire - all vying for the master control, which had become known as Merlin's Helm, a war ensued, which culminated in the defeat of the Empire and Dhal after the Cephlie volunteeringly took control of the Helm and wiped themselves out as well as the nanobot system.
Afterwards, Nyborg and Magnus set to peace talks, with suggestions that the struggling Empire might seek to house some of its citizens on the sparsely populated planet. The Doctor and his crew then quietly departed in his TARDIS. (PROSE: The Sorcerer's Apprentice)
References
Using nanomachines and a nova trigger, they attempted to force the empire's planets to stay together towards the end of the empire, though they were unsuccessful. (PROSE: The Dark Path)