Scottish serial killer Dennis Nilsen - who killed 15 men from 1978 to 1983 - sent the creepy letter to Taggart creator Glenn Chandler after learning he was staging a play which featured the killer's crimes.
The production, named Killers, is based on letters written by three of the most notorious serial murderers in Britain, including Nilsen, Yorkshire Ripper Peter Sutcliffe and Glasgow-born Moors Murderer Ian Brady from behind bars.
Actor Arron Usher, 36, of Johnstone, who portrays Nilsen in the show, said his blood ran cold when he heard Nilsen had contacted Chandler.
Arron, who also took over the part of the comic hero in this year's King's Theatre pantomime Sleeping Beauty, said last week:
"When I first heard Nilsen had been in touch, it was quite unnerving.
"A shiver ran down my spine and I was quite shocked he'd have the arrogance to get in touch."
Fraserburgh-born Nilsen contacted Chandler after hearing about the production of the show, writing a creepy letter which said: "I'm just a 68-year-old geezer."
Award-winning playwright and novelist Mr Chandler said: "Nilsen contacted me - he knew all about the show.
"He wanted me to know he was 'now just a 68-year-old geezer' who accepted that people would always want to write about him - which was his way of tacitly giving his approval to Killers.
"It was not direct. It was passed through another party.
"He knew so much about the play. It was almost as if he was sitting there watching it. It was actually beginning to spook me a bit."
Having debuted to rave reviews in Brighton last month, Killers will appear at the Assembly Rooms in Edinburgh throughout this summer's Festival Fringe.
It tries to uncover why ordinary people feel the need to enter into correspondence with serial killers - coming with the tagline: 'Enter the basement of The Old Police Cells Museum and come face to face with your worst nightmares.'
Former soldier Nilsen killed, defiled and dismembered 15 young men between December 1978 and February 1983, practically under the noses of his neighbours.
When police finally arrested him in 1983, it quickly became apparent that, had they linked a series of reported incidents from lucky escapees over the previous five years, they might well have halted his ghoulish killing spree considerably sooner.
He is currently serving a whole life term at maximum security Full Sutton Prison in England.