Exxilon was an ancient planet, significantly older than Earth. It orbited a star which was both larger and closer to the planet than the sun of Earth. Its sky was copper in colour. Due to the influence of the Great City of the Exxilons, it was a barren planet, covered in deserts, sand dunes and rocks. The seas had dried up, leaving behind few lakes. It was bitterly cold during the night and unbearably hot during the day. There was little in the way of life on the planet, and what little fruit was harvested was wizened because of the Great City. Exxilon was rich in parrinium, which brought both humans and Daleks to the planet. (PROSE: Death to the Daleks)
At some point in their history, the Exxilons decided to develop something that would outlast time itself, creating the Great City of the Exxilons. The City was intelligent and capable of repairing itself, allowing it to survive for thousands of years. However, the City turned against the Exxilons, and drove them from itself. It also drained all the energy from the planet, leaving it barren and desolate.
During the space plague, human and Dalek forces came to planet in search of parrinium, but were trapped when their ships were drained of power due to the Great City. The Third Doctor and Sarah Jane Smith also came to the planet by accident, and were likewise trapped. The Daleks modified their weapons and enslaved the Exxilons and humans to collect the parrinium, and sent Daleks to investigate the Great City. The Doctor also investigated the city and was able to stop it from draining energy. (PROSE: Death to the Daleks)
Exxilon was one of several planets where Daleks survived encounters with the Doctor. These survivors were classed as insane and taken to "intensive care" in the Dalek Asylum. The insane Daleks died when the asylum was destroyed by the Parliament of the Daleks. (TV: Asylum of the Daleks)
In the many years following these events, known as the Exxilon Gambit, (PROSE: The Secret Lives of Monsters) Earth established closer ties with Exxilon to secure more parrinium. When a second wave of the space plague struck, Earth began large scale parrinium mining on Exxilon, unaware that the Daleks had surreptitiously erected another power draining beacon to delay the humans' departure from the planet, its activation drawing the Fourth Doctor and Leela to the planet. Though the Doctor attempted to broker peace between humans and Exxilons, the Earth forces simply detonated the beacon as they took the Doctor and Leela prisoner. Anya Kingdom covertly had the Doctor's TARDIS loaded onto the human flagship as it left Exxilon. (AUDIO: The Dalek Protocol)