Nathemus

The Nathemus were a microscopic telepathic species which lived in volcanos.

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 prevents this by taking the TARDIS to Lakertya and deliberately flying into the Rani's tractor beam. The radiation in it force 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. (AUDIO: The Brink of Death)