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The Exxilons were an ancient humanoid species. Once a very technologically advanced culture, they mastered space travel and visited many planets while they were still primitive worlds, long before their respective native species reached the Exxilons' level of development.
However, the Exxilons outdid themselves with their own technological works, eventually building a monument beyond even their control. Their folly ushered in the destruction of their civilisation when it began killing them off in droves, causing them to greatly dwindle in numbers. The end consequence was their degeneration into a primal tribe society, after they surrendered to their own creation in fear and choose to worship it as a sort of god.
Following their descent to savagery, the Exxilons developed into two distinct sub-groups. They both had bark-like skin, inset eyes, flattened noses and wide mouths. They were both nocturnal and had large eyes adapted to the dark. One group of Exxilons were tall and powerfully built and lived on the surface of Exxilon. The other group were smaller, paler and lived deeper underground, with fluorescent markings on their bodies. (PROSE: Death to the Daleks)
The surface Exxilons had fallen further in development than the subterranean Exxilons and had little in the way of culture. They banned the use of advanced technology and any form of progress. They lived near the Great City, protecting it from intruders. They lived in the catacombs underground and would come out during the night. They were led by a high priest, who was the most sacred member of the tribes. The Exxilons would make sacrifices to the City, drugging prisoners and preparing them to be sacrificed in special temples deep underground. These rituals were long and complicated and if they were interrupted, they needed to be restarted.
The subterranean Exxilons lived deep underground, trying to stay out of the way of the surface Exxilons. They retained much of the knowledge of the ancient Exxilons and feared the City, but only as a danger instead of a god. (TV: Death to the Daleks, PROSE: Death to the Daleks)
The Exxilons were an ancient species, apparently capable of space travel before life had formed on other worlds. They travelled throughout the Mutter's Spiral and were reportedly the most powerful species in the universe, at least for a short time. They also influenced the development of other early civilisations, including the Earth. (TV: Death to the Daleks, PROSE: Death to the Daleks)
They were one of the species involved in the Millennium War, joining forces with the Uxarieans to fight the minions of the Mad Mind of Bophemeral in the Andromeda Galaxy. They were both caught in a trap that led to their eventual downfall and the creation of the Crab Nebula. (PROSE: The Quantum Archangel)
The Exxilons came to Earth during the early development of the Incan civilisation. They left their influences in their designs and did some unknown experiments on them. At the same time, they accidentally left behind the Xiuhcoatl and a ruptured fuel pod. Both of these were picked up by early Aztecs, and both had an influence on their civilisation. The fuel pod gave off radiation, causing mutations in the population and leading to the development of Huitzilin's powers. The Xiuhcoatl was also used by Huitzilin as a weapon of fear and terror, allowing him to control his citizens. (PROSE: The Left-Handed Hummingbird)
Eventually, the Exxilons decided to pool their knowledge and create the Great City of the Exxilons. It combined all of the science known to the Exxilons and was intelligent and capable of repairing itself. During this time, the Exxilons would travel to other inhabited worlds where they would construct great beacons on their surface that would drain the mental energy of the natives and send it back to the Great City. (AUDIO: The Exxilons) Eventually, the City decided that the only flaw left was the Exxilons themselves and it drove them out. The City drained all the energy from the planet, which meant the Exxilons were incapable of developing any technology above the stone age. They slowly descended into a primitive species.
During the space plague, humans came to Exxilon in search of parrinium. Their ship failed as soon as they reached the planet, and they were attacked by the primitive Exxilons as soon as they arrived. They tried to survive on the planet, but the Exxilons were a permanent danger. Shortly afterwards, the Third Doctor and Sarah Jane Smith came to the planet by accident, and Sarah was kidnapped to be sacrificed. Before the Doctor could do anything about it, the Daleks came to the planet, looking for parrinium which they could use to threaten the Earth colonies. They too were de-powered by the City, but after several defeats, they were able to modify their weapons, allowing them to fight back. They enslaved the surface Exxilons and forced them to mine the parrinium. While escaping from the Daleks, the Doctor and Sarah Jane encountered the underground Exxilons. The Doctor helped one, named Bellal, find a way into the City and destroyed it from within. (TV: Death to the Daleks)