Verdigris was a supernatural being.
Verdigris was a metallic green humanoid with burning eyes. He was immortal, indestructible, was able to animate trees and could change his appearance.
When Iris Wildthyme found out about the Third Doctor's exile to Earth by the Time Lords, she decided to free him. She went to Makorna and while drunk performed magic rituals to summon the spirits of the dead world. The result was Verdigris. After Iris was sober again, she forgot about summoning Verdigris.
Verdigris decided that the Doctor's work with UNIT was part of the problem, so he makes them forget that UNIT even exists. Following more complications, including impersonating the Master, Verdigris was finally unmasked and revealed his purpose to the Doctor. Ordered to undo the damage he had caused, Verdigris complied, but said he would continue trying to get the Doctor's imprisonment revoked.
Eventually, Verdigris entered the anti-matter universe to provoke Omega. With the defeat of Omega, the Time Lords ended the Doctor's exile. Verdigris appeared to the Doctor and Jo Grant to congratulate him on his freedom. His purpose fulfilled, Verdigris dissolved into a pile of rusted copper. (PROSE Verdigris)