Peter Allan, who was caged in 2009 for the slaying of dad-of-five Raymond Black, set upon Lyons at Perth Prison. Crime clan figure Lyons was jailed in 2010 for the road rage killing of van driver Mark Fleeman, 32.
Allan was accused of slashing Lyons on February 21, 2021. He was due to go on trial this week charged with assaulting Lyons.
Prosecutors claimed he struggled with him, kicked him and struck "him on his face with an unknown sharp implement to his severe injury and permanent disfigurement".
But Allan, 34, pled guilty to a reduced charge of punching Lyons, 42, on Thursday. He was handed a five-month jail term for the offence, to run alongside the life sentence he is already serving.
Allan was given a 15-year minimum term for the murder of Raymond Black, and is due to be released in May 2024. He was just a teenager when he killed Mr Black at the victim's home in Carluke, Lanarkshire, in January 2008.
Mr Black's two-year-old daughter was asleep in another room at the time of the murder. Lord Brodie said Allan had inflicted an "extraordinary level of violence" in the attack.
Allan had downed a cocktail of drink and drugs before the killing of Mr Black, which was so frenzied detectives described the crime scene as one of the worst they had encountered. Allan, also of Carluke, had denied murder, at first claiming Mr Black had attacked him.
But he later claimed the 48-year-old had made sexual advances towards him. The six day trial heard how Mr Black allowed Allan into his home and the two had been drinking together, while Allan had also taken up to 30 Librium tablets and smoked heroin.
Allan battered Mr Black with an Irn Bru bottle and a lamp then repeatedly cut and stabbed him with a large kitchen knife.
Medics discovered his throat had been cut through to the spine and a knife was left embedded just below his left eye socket.
He also landed back in the dock in 2022, after being caught with a makeshift weapon in prison. The chib was a four-and-a-half inch long piece of wood with 11 razor blades embedded in the end of it.
It was so big that it bulged in his pocket and was spotted by guards when he was acting suspiciously ahead of exercise at HMP Glenochil. He claimed he had the weapon as he was paranoid "everyone was out to get" him.
Lyons hit speeds of 100mph as he chased down his victim Mark Fleeman while driving on the M74 after a night out. He rammed the father-of-two's van, which left the road and overturned, resulting in Mr Fleeman dying at the scene
Mr Fleeman's passenger, 17-year-old Lee Allsup, also suffered life-changing brain and leg injuries. Lyons, who is the son of crime clan grandad David Lyons, was let out of jail in January this year to attend his father's funeral.
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He was one of a number of crime figures who paid their respects to 63-year-old and gave a speech as part of the eulogy before placing a rose in his dad's coffin.
The thug has also been caught with illicit mobile phones three times whilst serving his sentence, claiming he had the devices because he missed his family.
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