The ACT Magistrates Court heard on Tuesday that, by the end of the "appalling assault", human skin was stuck to the "shiv" Damien Glenn Featherstone had used on a man he wrongly suspected of being a paedophile.

An agreed statement of facts shows Featherstone, a former Brothers 4 Life chapter leader, launched the unprovoked attack in an exercise yard at Canberra's jail in November 2020.

The incident, which lasted about 40 seconds, began with Featherstone using the shiv to slash the victim across the left side of his face, causing the man to fall off a seat and onto the ground.

"[Featherstone] then stomped on [the victim's] upper body about six times, slashed him two times on the back of his head and neck area, stomped on the back of his head, kicked him in the face three times and stomped on his stomach, among other strikes," the agreed facts state.

"[He] was calling [the victim] a paedophile throughout the assault."

When prison guards started to intervene in the assault, Featherstone, 34, held the shiv "in a threatening motion" towards two of them.

"F--- off or I'll get you," Featherstone told the pair.

Correctional officers were eventually able to "herd" Featherstone and his co-offenders, including Kokomo's killer Frederick Tuifua, into a different yard so the victim could be treated.

"[Featherstone] discarded the shiv through a fence and it was later retrieved by the [correctional officers], who bagged it and allowed police to seize it," the agreed facts state.

"When police seized it, the shiv appeared to have a razor blade with blood on it and and what appeared to be human skin stuck to it."

The victim was cut, bruised and swollen by the attack, which also left behind "what appeared to be shoeprints on his face".

He did not wish to participate in an interview with police because he was "terrified of repercussions".

But the incident was caught on CCTV, and Featherstone pleaded guilty to four charges that included one of assault occasioning actual bodily harm.

The court heard on Tuesday that he had written a letter of apology to the bashing victim and the prison guards he had threatened with the shiv.

Featherstone admitted he had "mucked it up" in relation to the bashing victim, acknowledging that the man was not in fact a paedophile.

Prosecutor Trent Hickey remained unimpressed, summarising Featherstone's letter as effectively saying: "I thought you were a paedophile. Sorry, I got it wrong. Oops."

"It's utterly unacceptable," Mr Hickey told the court.

"The attack was unprovoked. It was uncalled for and it was unjustified."

The prosecutor called for "a lengthy sentence of imprisonment", noting Featherstone targeted vulnerable parts of the victim's body while armed and acting in concert with four others.

He drew attention to what magistrate Beth Campbell described as a "handsome" criminal history, which showed Featherstone had spent all but 450 days in custody since December 2004.

Mr Hickey said the 34-year-old had been released numerous times since that month, but 70 days was as long as he had lasted before being locked up again.

"Because of this offender's past and his entrenched criminal attitudes, his prospects of rehabilitation are hopeless," the prosecutor told the court.

Featherstone's lawyer, Jacob Robertson, conceded the 34-year-old's criminal history and the latest assault were both "appalling".

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Mr Robertson highlighted Featherstone's disadvantaged childhood and a psychologist's opinion that the 34-year-old had become institutionalised.

The lawyer also said Featherstone, a schizophrenic, had a four-year-old son he had never seen outside the Alexander Maconochie Centre.

At the time of the attack, Featherstone was serving a sentence of more than seven years behind bars for his role in an ice-fuelled crime spree across Canberra.

That jail term does not expire until 2025.

He will be sentenced over the prison bashing at a later date, with proceedings due to resume on August 15.

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