Deathworm Morphant

Deathworms or Morphants were symbiotic creatures used to survive beyond death.

History
One account stated that Deathworms were created by the Morgs. (EDA: The Eight Doctors) However, another account noted that they were created on the planet Skaro. (DWM: The Fallen)

The Master used a Deathworm (or morphant) to survive his execution by the Daleks. He managed to interfere with the Doctor's TARDIS to send it to the 30 December 1999. In the form of the Deathworm, the Master slithered out of the TARDIS through its keyhole and across the pavement. The Seventh Doctor saw this but was unable to warn Chang Lee. Later that night the Master infiltrated the house of Bruce, the paramedic who attended to the wounded Doctor. He slithered down his throat while he was asleep and possessed him, gaining a new body. (DW: 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 be Time Lord DNA. As it happened, it was deathworm DNA, and the head scientist, Eustace Stark, attempted to include the Deathworm's abilities into 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 deathworm's hunger. Transformed into a huge deathworm dwelling in London sewers, he kept insisting he was in full control of himself. He ate people and absorbed their knowledge. 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 deathworm hybrid by slamming a combat helicopter onto it. (DWM: The Fallen)

Behind the scenes
The creature was never called a Morphant during the 1996 TV Movie. The name appeared only in the script and in preproduction documentation, but it did appear in the later Doctor Who Magazine comics story The Fallen.

The BBC Eighth Doctor Adventures novel The Eight Doctors by Terrance Dicks describes the creature as a "Deathworm", which implies implying that The Eight Doctors and The Fallen don't take place in the same continuity.