Valis

Valis was "high arbiter of the darkness". His goal was to have "dominion over all things". He was a disembodied being composed of gas-suspended carbon particles and was highly flammable.

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

About a fortnight later, Valis travelled in a spaceship across a couple of parsecs to Research Base Truro, transmitting security protocols to bypass the quarantine. There, he joined forces with several aliens the Doctor had defeated before to try to kill the Doctor. He was shot by Zargath, who believed him the Doctor in disguise, and exploded in the face of his murderer. (COMIC: Death to the Doctor!)

Behind the scenes
In an interview with Jonathan Morris released in The Widow's Curse the description for Valis was given as; "cowled in robes like the Ghost of Christmas Yet To Come. He should look scary and enigmatic".