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 indirectly killed commanding officer Jonathan Moran, due to trying to attack Richard Pritchard and accidentally turning on his ship's engines, then transformed him into a ghost. He later watched as Moran's ghost killed and turned Pritchard into another ghost by allowing water to flood the chamber. He and Moran also adjusted the Drum's settings of day and night so they could continue their attack and also called for a evacuation team to have more people to turn into transmitters.

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

When the Doctor travelled back to a military base in 1980 he encountered Prentis, who revealed he was an Undertaker. Prentis was transporting the body of the Fisher King, who had conquered Tivoli before being defeated, to Earth. However it was revealed the Fisher King wasn't dead and he killed Prentis. (TV: Before the Flood)

Behind the scenes
Prentis is unnamed in Under the Lake. He is, however, named in the episode credits and in Before the Flood.