Metamorphic symbiosis regenerator

A metamorphic symbiosis regenerator was a device 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 hope of becoming immortal, but instead were forced to suffer unending and forceful regenerations. Their undead-like condition were so severe that Mawdryn and his colleagues longed for death, and attempted to persuade the Fifth Doctor to sacrifice his remaining regenerations so they could die. Fortunately, it was the energy discharge from the two versions of the Brigadier meeting which cured them of their undead state instead of the former. (TV: Mawdryn Undead)