Basement Entity

The unidentified Shapeshifter V known as the Basement Entity was one of a race of spirit-like creatures who feasted on the life force of other beings that it copied the physical form of.

Biography
Most of the Shapeshifter V species were thought to be extinct after a violent assault on their homeworld. At least one survived, however, and was hired by Scooter 2000 to capture or kill the Doctor and gain access to the secret of the the TARDIS. Once it copied the Doctor's form, it trapped him in the basement with it and slowly drained him. The Doctor played a special tune with his sonic penny whistle, however, and sent the creature back into its corner. It drained of energy, lost the form it copied and quietly died moments later. (Horror at Towell Farm)

Trivia

 * The Diet Coke stain caused during the scene was a mystery to the family until Doherty left two days later, finally admitting it was him who dropped it.
 * Doherty almost injured himself falling over because instead of taking his trousers off he simply put them down to his ankles.