John Martin helps take Anthropoid to the cinema screen
Nicknamed "The Hangman of Prague", SS general Reinhard Heydrich was a man even Hitler himself feared as "the man with an iron heart".
Heydrich - one of the architects of the Final Solution when he was governor of occupied Bohemia - was assassinated by British-trained Czech and Slovak troops in May 1942.
It was an operation which was so audacious in its planning and execution that it inspired John Martin from Mold, Flintshire, to research the topic for more than 40 years.
He interviewed Heydrich's son, and descendants of the Special Operations Executive hit-team which killed him, as part of a book which has now been made into a film.
Anthropoid, named after the mission's codename, has just gone on general release.
It stars actors Cillian Murphy and Fifty Shades of Grey star Jamie Dornan who portrays a Czech resistance fighter.
Mr Martin, who is an extra in the film, said: "Not many people have heard of Anthropoid, partly because it was earlier on in the war, and partly because afterwards Britain's role was played down by the Soviets.
"It took me until after the fall of the Iron Curtain to be able to get into Prague and properly investigate what happened."
Born in 1904, Reinhard Heydrich was a decorated German naval officer in the Weimar Republic.
During the rise of the Nazis he progressed quickly to head several branches of the Nazi secret police, including the Gestapo and SD, and was a key figure in both 'The Night of The Long Knives' and 'Kristallnacht'.
As Nazi head in occupied Czechoslovakia, he was instructed to improve the productivity of the region's flagging munitions and automobile factories.
It was a role which he performed so successfully that the Allies determined he must be removed.
Mr Martin said: "Before Heydrich's appointment the Czech Resistance had been quite successful in reducing productivity by as much as a third through go-slows.
"Heydrich introduced a policy he called 'Sugar and Whip', whereby those who cooperated gained extra rations and clothing, whilst those who didn't were kidnapped, tortured and killed.
"In one way the fact he rewarded people might be seen as a humanising feature, but I think it makes him all the more chilling.
"For him, it wasn't about the violence, he'd do whatever it took to achieve the Nazis' ultimate ambitions."
In December 1941 the SOE assassination squad of Josef Gabcik and Jan Kubis was parachuted into Bohemia from an RAF Halifax bomber.
They spent six months undercover planning their assault, before finally ambushing Heydrich on a blind hairpin bend on the Prague to Dresden road on 27 May 1942.
Heydrich died of his injuries a week later, and although the Nazis reaped devastating revenge, Mr Martin contends it was still a worthwhile mission.
Mr Martin said: "Hitler personally ordered the levelling of two villages - Lidice and Ležáky - where all the men over 16 were shot and all the women transported to concentration camps.
"The ordinary population paid a terrible price for what was essentially an economic and military attack. Yet you can see in Hitler's response just what a devastating blow the assassination was.
"Hitler was furious at the arrogance which led Heydrich to travel in an unguarded open top car, and later he'd say that the loss of Heydrich was the beginning of the end in the battle for the Eastern Front."
Mr Martin currently works on cruise ships, where he gives both comedy performances and history lectures on Heydrich.
"It's a strange life for me," he said. "One night I'll be telling jokes, and the next afternoon I'm lecturing on the holocaust.
"Some comedians can make jokes about things like that, but I have to keep the two completely separate.
"I'm delighted that my book's been made into a film. I can tell people about Heydrich, but it's only now they'll see him on the screen, that his true horror will really sink in."