Paedophile priest Father Brendan Smyth threatened to punch another cleric when warned to stay away from altar boys, an inquiry has heard.
Fr William Fitzgerald, who served with Smyth at Kilnacrott Abbey in Co Cavan during the late 1980s, described the serial child molester as a scary individual whose notoriety extended across the world.
Giving evidence to the Historical Abuse Inquiry (HIA) the 61-year-old Australian said he had to bar Smyth from church rehearsals with nine altar boys.
"I told him, 'in view of the rumours about you and your sexual activities with children, you will not be anywhere near these kids under my watch'," he said.
"He said 'I'll knock your head off.'
"I said 'oh make my day you b*****d. I'll knock your head clean off'."
Retired judge Anthony Hart is leading the HIA probe, one of the UK's largest inquiries into physical, sexual and emotional harm to children at homes run by the church, state and voluntary organisations.
Smyth, who was at the centre of one of the first clerical child sex abuse scandals to rock the Catholic Church, was eventually convicted of more than 100 child abuse charges on both sides of the Irish border. He died in prison in 1997, after serving just three years behind bars.
Apologising to victims, Fr Fitzgerald slammed efforts to stop the paedophile as "pathetic". He also claimed Smyth's poisonous legacy had forced the closure of Kilnacrott Abbey and destroyed the Norbertine order.
Fr Fitzgerald voluntarily attended the hearing at Banbridge Courthouse in Co Down on behalf of the Norbertines.
He was first made aware of Smyth's reputation while talking with a colleague in Perth, Australia in 1973.
Meanwhile, the inquiry also heard how Smyth frequented children's homes in Belfast and Co Down as well as grooming boys and girls for abuse after befriending their parents.
Fr Fitzgerald said the boot of his car was always filled with "candy". "That was his tool in dealing with children," he said.
Earlier it emerged that gardai had known about Smyth's activities in the 1970s.
Confidential documents from St Patrick's psychiatric hospital in Dublin revealed he had asked to be admitted after coming to the attention of gardai in 1973.
In a letter to an officer at Finglas Garda station dated November 1, 1973, Smyth's psychiatrist wrote: "I have been asked to write to you by Fr Brendan Smyth of Holy Trinity Abbey, Kilnacrott.
"He has been a patient under my care for some months and I am familiar with the nature of his problems. I am writing to his superiors suggesting that he should have a period of in-patient care in St Patrick's Hospital or in St Edmonds Bury.
"I hope this arrangement will be satisfactory to you and your superiors."
Joseph Aiken, counsel for the inquiry, said Smyth had been conducting a retreat in Finglas in July 1973.
"For some reason Brendan Smyth has asked the doctor looking after him to write a letter to Finglas Garda station to say that he is going to be taken in for some in-patient treatment," Mr Aiken said.
The documents, which were only released to the inquiry today despite repeated requests by the Norbertine order over decades, also revealed how Smyth was officially diagnosed as a paedophile in 1974.
A case summary dated February 1974, read: "Psychosexual difficulties for many years. First developed in the Novitiate. A recurring problem no matter where he has been stationed. His paedophilia has brought him into contact with police."
Tomorrow, the inquiry is expected to hear oral evidence from retired Catholic Primate Cardinal Sean Brady who was a note-taker during a much criticised Church investigation into Smyth's deviant behaviour in 1975.