Vol Mercurius was a human who lived during the 26th century.
He owned the planet Dispater, having bought it with money he had stolen from Abslom Daak. He only had the robot Klikbrain as a companion. Daak went to Dispater to recruit Mercurius to fight the Daleks. They were invading the planet when Daak arrived, which helped convince Mercurius to depart on the Kill-Wagon. He left Klikbrain on Dispater. (COMIC: Star Tigers)
During a battle against the Daleks over the planet Hell, the Kill-Wagon was badly damaged and forced to crash land, killing Mercurius, Salander and Harma and leaving Daak as the last survivor. Their corpses were found by the Seventh Doctor. (COMIC: Nemesis of the Daleks)
Later, Mercurius was revived along with Salander and Harma, having been healed by the medicine of the native Helkans. They resided in a camp with Taiyin's cyrogenic freezer-unit when they were found by the Seventh Doctor and Bernice Summerfield, who were intercepted by Daak. The camp was then transmated away, bringing the party to Daak's seemingly human benefactors, who promised to revive Taiyin in return for the Doctor. However, the Doctor revealed that the benefactors were actually robots used by the Daleks, and that they had been transported to Skaro. Fighting their way through the Daleks loyal to the Emperor Dalek, Daak's party found Taiyin's body, only to be rendered unconscious by gas emitted from her mouth.
While the Doctor was taken for interrogation, the crew and Benny were conditioned to serve the Daleks, their lives used as leverage to force the Doctor to accompany the Daleks to Davros' rebel base on Spiridon. A battle between the Dalek factions ensued, during which the Doctor freed his allies, who fought the Daleks alongside the native Spiridon. The Doctor and company were eventually captured by Davros, who had them accompany him back to Skaro, where he saw to the conquest of the planet and his establishment as the new Emperor Dalek, having his predecessor exterminated.
Believing Taiyin to still be on Skaro, Daak, after badly wounding Davros, was intent on rescuing her. When Daak insisted on staying behind, the Doctor had his crew force him into the TARDIS. Following this ordeal, the Doctor took the crew to a bar on the planet Paradise. (COMIC: Emperor of the Daleks!)