The Monitor

The Monitor was a Logopolitan and served as the interface between visitors and the other Logopolitans.

The Monitor was a good friend of the Doctor, swiftly agreeing to help him by using the Logopolitans' skills in Block Transfer Computations to repair the TARDIS' chameleon circuit, but he still kept Logopolis' secret from him at all times. When arrived on Logopolis in 1981 and silenced Logopolis, the secret was revealed: the Logopolitans had been staving off the end of the universe, and now that they were dead, entropy would build up and destroy the universe.

In a final attempt to gather all information on programming the Charged Vacuum Emboitments, which had prevented the end of the universe, the Monitor was disintegrated by the entropy. (TV: Logopolis)

Magus, formed by Nyssa's experiences and interpretations, had the Monitor's physical form. (AUDIO: Psychodrome)

Behind the scenes
Harry Andrews, Bernard Archard, Geoffrey Bayldon, Peter Cushing, Maurice Denham, Frank Finlay, Barry Foster, Marius Goring, Michael Gough, William Lucas, Nigel  Stock, Frank Windsor and Peter Wyngarde were considered for the role before John Fraser was cast.