Valis

Valis was a villain whose goal was to have "dominion over all things". He was a disembodied being composed of gas-suspended carbon particles and highly flammable.

He captured the Doctor and Izzy after having stole the Crystal of consciousness. However, the Doctor had swapped the crystal with an piece of perspex and Valis was defeated.

Two weeks later, Valis joined forces with several alien to try to kill the Doctor, their common enemy. He was shot by Zargath, who believed him the Doctor in disguise and exploded in the face of his murderer. (DW: Death to the Doctor!)