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 a deathworm from them and consumed it to survive his execution on Skaro by the Daleks. (EDA: The Eight Doctors)

In the form of a deathworm, the Master escaped from the Doctor and possessed the body of the ambulance driver Bruce. (DW: Doctor Who)


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