Albar Prentis

Albar Prentis was a Tivolian undertaker who was killed in 1980 and returned as a ghost at the underwater mining base, the Drum, in 2119.

History
When the Doctor traveled back to a military base in 1980 he encountered Prentis, who revealed he was a funeral director. Prentis was transporting the body of the Fisher King, who had conquered Tivoli before being defeated, to Earth. He asked if the Doctor wanted to oppress him. However it was revealed the Fisher King wasn't dead and he killed Prentis to turn him into a transmitter. (TV: Before the Flood)

Prentis' ghost appeared after his ship was recovered by 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)

The Doctor later told Clara that UNIT would cut out the Faraday cage and eject it into space where away from the Earth's magnetic field, the ghosts would dissipate. (TV: Before the Flood)

Behind the scenes
In Under the Lake, Prentis is not named on-screen, only in the credits. His full name is given only in Before the Flood.