Dirk Slipstream
Dirk Slipstream was a criminal who served time in Volag-Noc after being caught by the Doctor in an earlier incarnation. He was sentenced to fifty years per person he killed duringNinth Doctorst- that's 600 dead working out to about 32000 years. In Night of the Humans, he went searching for the Mymon Key- an object of unlimited power- on the Gyre. Slipstream was presumably killed by carnivorous Sollogs while making one last desperate bid for the Mymon Key.
Notes
Precisely what body the Doctor was in when he last caught Slipstream was never specified. The Eleventh Doctor reflected that it had been 'years and regenerations' since he'd last seen Slipstream, but given Slipstream's knowledge of the Time War, the Eighth or Ninth Doctors seem the most likely candidates.
