Terrence Hardiman has died aged 86, his agent has said.
The actor - best known as the lead role in The Demon Headmaster - also appeared in TV series' such as The Crown, Doctor Who and Wallander. The legendary star is remembered fondly by a generation of kids who were petrified by his performance in the CBBC series, which first ran from 1996 to 1998.
Based on the children's books of the same name by Gillian Cross, The Demon Headmaster saw Hardiman as a frightening figure.
He kept his piercing green eyes largely hidden, creepily, behind dark glasses, only removing them to hypnotise his victims after telling them: "Look into my eyes."
The children's drama ran between 1996 and 1998 but Hardiman reprised his role in 2019, when a direct sequel of the original series began with Nicholas Gleaves taking over as the iconic character.
But despite playing a bad guy on screen, Hardiman was thought to be a lovely gentleman to deal with when the cameras stopped rolling.
Born in 1937 in Forest Gate, Newham, London, Hardiman studied English at Fitzwilliam College, Cambridge, where the late actor got the acting bug.
Hardiman performed for The Marlowe Society and the Cambridge University Amateur Dramatic Club, alongside many of his future screen co-stars such as Derek Jacobi.
Throughout his illustrious career, Hardiman appeared in a number of popular TV shows including The Bill, Prime Suspect and Juliet Bravo, and often played figures of authority.
He was regularly praised for his deliverance of a powerful performance.
Tributes have flooded in for the late star.
"Awfully sad news about Terrence Hardiman, who has passed away aged 86. Terrified a generation as the Demon Headmaster, but could not have been more of a good-natured gentleman in real life," one fan wrote on Twitter following the actor's agent announcing his death.
Another wrote: "Sad news, as he was a big part of my childhood.
"But also, I can't believe how this is the way 90% of people my age are finding out the name of the actor who played The Demon Headmaster."
A third added: "A brilliant actor, obviously great as The Demon Headmaster and also terrific in the final series of Secret Army.
"Allegedly he kept the Headmaster's glasses prop after filming, because he thought it would be a character people would want to see him return to."
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