Albar Prentis

Prentis was a Tivolian who at some time died and was transformed into a ghost in the underwater mining base, the Drum, in 2119.

He killed commanding officer Jonathan Moran and transformed him into a ghost. He later did the same to crewmember Richard Pritchard.

After the Drum crew discovered that all three ghosts were all muttering something, the Doctor led the ghosts into the base's Faraday cage, where he trapped them with the intention to discover what they were muttering. (TV: Under the Lake)

Behind the scenes
Prentis is unnamed in the episode. He is, however, named in the episode credits.