When the planet Xeraphas was devastated by crossfire during the Vardon-Kosnax war, the inhabitants of the planet - Xeraphin - travelled to prehistoric Earth in their spaceship, hoping to build a new home. But the radiation that had destroyed Xeraphas had also contaminated them, and upon their arrival on Earth, they discovered that they were all sick with radiation poisoning. The Xeraphin abandoned their ship and melded themselves into a single unity using their psychic power. They ensconced themselves within a sanctum, planning to rest until the radiation contamination had passed, at which point they would regenerate.
However, as the moment of their regeneration neared, the Master also arrived on prehistoric Earth, his TARDIS having been damaged enough to leave him stranded there. In need of a new power source, the Master was irresistibly drawn to the Xeraphin sanctum and its power. At the moment of their regeneration, when the first-reborn Xeraphin emerged, the Master destroyed them with his Tissue Compression Eliminator. The Xeraphin retreated back to their resting place, and were apparently able to expel the Master from their sanctum.
Through the projection of his mind, the Master was able to communicate with the baseness of the Xeraphin being, but his intervention caused a schism in the Xeraphin nature that infinitely divided them into a good half and an evil half. The good half of the Xeraphin, which aided Tegan and Nyssa and resisted the Master, wanted to defeat the Master and become corporeal. The evil half of the Xeraphin, which had been aiding the Master, wanted to have its power used by the Master and feared by all others. The side that won would gain control of all the Xeraphin power.
Within the Xeraphin sanctum is a large sarcophagus containing the neuronic nucleus, which is an immeasurable intelligence at the center of a psychic vortex. Its appearance as a fuzzy orange blob belies its all-seeing, all-knowing power. This creature, the physical amalgamation of an entire race into one organism with one immense personality, contains the wisdom of the universe.
The Xeraphin are highly psychic beings, able to wield psychokinetic energy, possess people in order to communicate, exert a hypnotic influence over people, and telepathically project components of their gestalt - such as people who have been absorbed into it - into other locations, even the interior of a TARDIS. The power of the Xeraphin is controlled by the two aspects, good and evil, of the same personality, and the good Xeraphin use their power to protect Nyssa and Tegan. The Master is able to access the Xeraphin power to perform what seem to be conjuring tricks, as well as to create mass hallucinations, but the schism within the Xeraphin means that the power is unstable - sometimes it works for him, but sometimes it works against him. And if the sarcophagus is damaged then the Xeraphin's power is also cut off until they can effect repairs.
When the Doctor and his party become trapped in the Xeraphin sanctum, the Xeraphin advise them that they will need knowledge to escape. To give them this knowledge, the Xeraphin will have to manifest in a corporeal body, and this requires a sacrifice. Professor Hayden, one of the passengers on board Concorde, volunteers to be absorbed into the Xeraphin lifeforce and gain their knowledge, and the Xeraphin energy consumes him, breaking up his molecular structure and turning his body into Plasmaton material. With this done, the Xeraphin are able to materialize.
Both black and white Xeraphin are able to manifest themselves in order to communicate with other beings and with each other. The chosen representatives are Anithon, of the light Xeraphin, and Zarak, of the dark. Zarak tries to summon more dark Xeraphin, including Kalistoran, Vaan and Mordal, and Anithon urges the TARDIS crew to will them not to appear. The focused concentration of their willpower is effective, and the dark Xeraphin are repelled.
The Master is able to use an induction loop to transfer the Xeraphin sarcophagus to the center of his TARDIS, but some tampering by the Doctor leads to his materialization on the planet Xeraphas, where his TARDIS again gets stuck. (He is later able to escape from Xeraphas using the shape-changing android Kamelion, which had been a tool of the aliens who had earlier invaded the planet.) Because the radiation has cleared from Xeraphas by this time period, it is safe for the Xeraphin to regenerate and repopulate the planet.