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 they were forced to suffer unending and forceful regenerations. Their undead-like condition was 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. This was ultimately not necessary, as the energy they needed came instead from the Blinovitch Limitation Effect caused by two versions of the Brigadier meeting. (TV: Mawdryn Undead)