Quoth

The Quoth were a subatomic sentient species, no larger than a string of quarks.

They lived at least eighteen thousand times faster than a human. Their original homeworld was a spinning cylinder of neutronium, a light-day long, whose rotation distorted time. When the cylinder was destroyed in a cosmic accident, the Quoth drifted through space until they encountered Earth.

There, they found the only source for their birthing materials was inside the human brain, created as a side-effect of psionics. Ironically, the extraction process stimulated the psychic nodes in the host's brain, increasing their psionic ability.

Once freed from Montague's enslavement, the Seventh Doctor took them to a new home, the neutronium remains of a first-generation star at the galactic core. (PROSE: The Death of Art)