Deathworm Morphant

Deathworm Morphants, sometimes just called deathworms (PROSE: The Eight Doctors) or morphants (COMIC: The Fallen) were symbiotic creatures used to survive beyond death. (AUDIO: Mastermind)

An individual swallowed a deathworm before going into danger, where it became dormant and lived inside their body. If the individual was killed, the deathworm absorbed their essence, body and spirit, and lived on in their remains, even in ashes if the body was burned. As soon as it could, the deathworm would seek a new host, taking over the host and then dying, allowing the spirit to live again in a new body. (PROSE: The Eight Doctors)

History
One account stated that Deathworm Morphants were created by the Morgs and resided on their home planet, (PROSE: The Eight Doctors) while another stated that they were indigenous to the planet Skaro. (COMIC: The Fallen) The Master learned about them upon breaking into the Matrix. (PROSE: The Quantum Archangel) Scheming to steal the Doctor's body so as to gain his regenerations, 's "final incarnation" made his way to the planet of the Morgs, where he bought a jar full of Morphant specimens from them. He then experimented on the Morphants to give them additional abilities and allow them to bear the weight of an entire Time Lord mind. Only one, impossibly-strengthened Morphant survived the Master's experiment; he ingested it and then arranged to have his now-disposable physical form be killed by the Daleks. (PROSE: The Eight Doctors)

In another account, however, the Eighth Doctor stated that he himself could take on the form of another species upon his death, and only then, quite like his fellow Time Lord the Master — and was not surprised to see the Master had taken such a form. (TV: Doctor Who)

At any rate, did survive his execution by the Daleks in the form of a Deathworm Morphant, who slithered out of the casket containing his humanoid body's ashes through the keyhole. He managed to interfere with the Doctor's TARDIS to send it to 30 December 1999. In this form, the Master slithered out of the TARDIS through its keyhole and across the pavement. Later that night, the Master infiltrated the house of an American paramedic named Bruce and slithered down his throat while he was asleep, killing him in the process of gaining. (TV: Doctor Who)

Using a sample of the substance the Master had spat at her, cardiologist Grace Holloway initiated an ambitious project to imbue humans with regenerative abilities, believing it to contain Time Lord DNA. However, it was in fact Morphant genetic material, and therefore contained Morphant DNA. The head scientist of the project, Donald Stark, attempted to add the Morphant's abilities to his own genetic makeup. As he lacked the mental powers of the Master, he failed to retain his own identity and was left a slave to the Morphant's hunger.

Transformed into a huge Morphant dwelling in London sewers, Stark kept insisting he was in full control of himself. He ate people and absorbed their knowledge and identities; this backfired when it turned out that he lacked the ability to suppress the many minds absorbed by the Morphant. Izzy Sinclair nearly fell prey to its carnivorous desires, though upon detecting she had contact with the Doctor, she was left alive as a bargaining chip. The Doctor destroyed the Morphant hybrid by slamming a combat helicopter onto it. (COMIC: The Fallen)

Behind the scenes

 * The creature was never named on-screen. It was only called a "Morphant" in the script and in preproduction documentation, but the term was later used in the Doctor Who Magazine comic story The Fallen. The BBC Eighth Doctor Adventures novel The Eight Doctors by Terrance Dicks describes the creature as a "Deathworm". Mastermind gave clarification on the name of the creature, calling it a "Deathworm Morphant", which Planet of Dust later corroborated.