Vortex Wraiths were chaotic, quasi-biological life forms indigenous to the Time Vortex.
The Time Lords built defences into their TARDISes to prevent the Wraiths from noticing them. (PROSE: The Slow Empire)
After Gallifrey was destroyed, the Wraiths were forced to evacuate the Time Vortex due to the Council of Eight. (PROSE: Anachrophobia)
The transference network of the Slow Empire once allowed a group of Wraiths to penetrate the Doctor's TARDIS. (PROSE: The Slow Empire)