Vassarian

 was a Time Lord soldier whose Battle TARDIS was damaged by a Dalek attack. He managed to eject his stasis pod from the TARDIS whilst it repaired itself but he started to regenerate and was placed in stasis without completing. He was leaking artron energy which the Sontarans weaponised. He made contact with the Doctor to tell him how to stop the Sontarans. (AUDIO: The Eternity Cage)