Metamorphic symbiosis regenerator

A Metamorphic Symbiosis Regenerator was made by the Time Lords to cure one of their own in the case of acute regenerative crisis. One was stolen by Mawdryn and his accomplices in the vain hope of becoming immortal, instead they were force to suffer unending and forceful regenerations. Their undead-like condition were so severe, that Mawdryn and his colleagues longed for death, thus opting to persuade the Doctor to sacrifice his remaining regenerations to die. Luckily, it was the energy discharge from the two versions of the Brigadier that cured them of their undead state instead of the latter. (DW: Mawdryn Undead)