The Nathemus were a microscopic telepathic species which lived in volcanos. They fed on telepathic energy.
The Valeyard saved them when their home on Plestinious' moon was going to be converted into a power plant. Gaining their loyalty from this, he put them in the symbiotic nuclei of the Doctor's TARDIS. He wanted to use them to take over the Matrix, gain control of Gallifrey and destroy the Sixth Doctor.
However, the Doctor prevented this by taking the TARDIS to Lakertya and deliberately flying to the source of the radiation, coming under fire with irradiated lasers being used by the First Rani. The radiation in them forced the Time Lord to regenerate, freeing him and the TARDIS from the mental influence of the Nathemus. This killed them, since they needed his unique mind to survive, foiling the Valeyard's scheme. (AUDIO: The Brink of Death)