Deathworm Morphant

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

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

The Master used a deathworm/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 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 and slithered down his throat while he was asleep, killing the paramedic while gaining his second non-regenerative incarnation. (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 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. (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", which seems to imply that The Eight Doctors and The Fallen don't take place in the same continuity.