Deathworm Morphant

Deathworms were symbiotic creatures used by the Morgs to enable them to survive beyond death.

Overview
When a deathworm was consumed, it would incubate in its host's body until the moment of death, then leave with the host's consciousness preserved inside it and seek out a new host. It could survive most form of abuse, including disintegration. The Master sought out the Morg to obtain one from them and consumed it to survive his execution on Skaro by the Daleks. (EDA: The Eight Doctors)

This explains the Master's survival after his execution in a new incarnation, in which in this new form, he possessed the body of the ambulance driver Bruce. (DW: Doctor Who: The TV Movie)


 * Another account (DWM: The Fallen) claims that the Master used a morphant from Skaro to effect this.