Mass murderer Robert Maudsley, who is serving a life sentence for brutally killing four people, has warned he'll kill again if he's ever released.
The monstrous killer, who is being held in Wakefield Prison, has already spent 40 years living in solitary confinement in a glass cage after he was deemed too dangerous to be around other prisoners.
But although he's previously complained his interment reminded him of a traumatic childhood, it has been reported he's now quite content living in the underground box.
Speaking in a new documentary 'HMP Wakefield: Evil Behind Bars' Maudsley's nephew Gavin said his uncle is perfectly happy.
He added his softly spoken, well-read uncle would kill again if he was ever released back into society.
Maudsley, from Toxteth, Liverpool, was just 21 when he committed his first murder in 1974.
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The serial killer, who had been working as a rent boy at the time, butchered one of his clients, John Farrell.
The murder was so violent, cops named him "blue" because of the colour of his face.
Maudsley was arrested and eventually convicted of murder. He was jailed with the recommendation he should never be released.
When he was first locked up Maudsley was sent to Broadmoor Hospital, which houses some of the UK's most violent prisoners.
After three relatively quiet years behind bars, the serial killer seized an opportunity to attack child molester David Francis in 1977, with the help of fellow prisoner David Cheeseman.
The pair barricaded themselves in a cell with Francis before spending the next nine hours brutally torturing the sex offender to death.
During the attack, Maudsley shoved a spoon so far into Francis's ear, it wedged into his brain.
After the killing, for which he was convicted of manslaughter, he was deemed too dangerous to stay in Broadmoor and was transferred to Wakefield Prison.
But Maudsley's streak for vigilante violence continued and after he'd settled into his new cell the following year, he strangled and stabbed wife-killer Salney Darwood and hid the body under his bed.
Thirsty for more blood, he hunted prison corridors for his next victim - Bill Roberts, who had been jailed for sexually abusing a seven-year-old girl.
He stabbed Roberts to death before hacking at his skull with a makeshift dagger.
After the bloodshed, Maudsley calmly walked up to a prison guard and chillingly said there would be two fewer people at dinner that night.
His rampage rung alarm bells for prison bosses who deemed him too dangerous to mix with the general prison population and a special cell was constructed to keep him in.
The cell, which was completed in 1983, was dubbed the glass cage - as it resembled Anthony Hopkins' cell in Silence of the Lambs.
At 5.5mx4.5m, it's surrounded by bulletproof glass which prison guards peer into to keep a close eye on him.
The only furniture is a table and a chair, which are both made of compressed cardboard, while his toilet and sink are bolted to the floor.
The door of the cell is made of solid steel, which opens into a cage just inside. Maudsley's bed is a concrete slab.
The see-through walls contain a slit, through which guards pass him meals and other things he needs.
Spending his days entombed in the cell, Maudsley is allowed one hour of exercise per day. When he goes to the exercise yard, he's escorted by six guards and he never has access to other inmates.
When interviewed, Maudsley said he felt "tortured" in solitary confinement and claimed his speech has suffered from never speaking to anyone.
He said: "I feel no officer takes any interest in me and they're only concerned with when to open the door and then to make sure I get back in my cell as soon as possible.
"I think an officer could stop and talk a bit but they never do and it's these thoughts that I think about most of the time."
But in a letter to Gavin, 40, he claimed he is "happy and content" in the box and has warned killing will be on his mind if he's ever released.
Maudsley has claimed the confinement has given him flashbacks to his childhood, when he was regularly locked away and beaten.
The serial killer had an abusive childhood at the hands of his father. After being dumped in an orphanage as an infant, his parents brought him and his 11 siblings home when he was eight.
He was regularly hit and often took extra beatings to protect his siblings. Once, he was locked in a room for six months, with his only human contact coming from beatings from his dad.
In the year 2000 Maudsley asked for the terms of his imprisonment to be relaxed - begging for a pet budgie or, if that request was denied, a cyanide capsule so he could end his life.
Both requests were turned down, leaving him to live out his days in the glass cell underneath Wakefield Prison.
