Steven Souley, 26, and his accomplice Zico Malavin, 20, were remanded in custody after they admitted attacking Robert Eadie, 25, in Glasgow in 2014.
The victim required surgery to repair a wound to his groin and penis after being hit with an axe and knife.
Souley was previously jailed for seven years for killing a man in 2010.
Sentence on both accused was deferred at the High Court in Glasgow after they admitted assaulting Mr Eadie to his severe injury and permanent disfigurement.
The court heard that the attack happened at a Christmas party hosted by Mr Eadie's sister at her flat in the city's Summerston area.
Souley and Malavin, who were not invited, turned up at about midnight. Souley was armed with an axe and Malavin - who was wearing a red Christmas jumper - was clutching a knife.
Mr Eadie's sister, who knew the pair, demanded that they leave. Her brother was attacked after he tried to put them out of the flat.
Souley and Malavin both fled the scene and police and paramedics were called.
A knife and a Christmas sweater were later found at a property linked to Souley.
It was months later that the pair were charged.
The court heard that Mr Eadie needed an operation to repair a "deep wound" to his groin and to his penis. He also suffered injuries to his head and hand.
He was released from hospital days later, but will be permanently scarred.
Souley and Malavin, both from the city's Drumchapel area, will be sentenced next month.
Souley was jailed in 2011 after being convicted of the culpable homicide of 41-year-old Andrew Curran.
In May 2014 he was deemed safe enough to be freed early.
Zico Malavin's brothers were both found guilty of murder over the attack on Mr Curran.
Angus Malavin was jailed for 18 years and younger brother Zak for 17 years.
Mr Curran died from a sword wound to the neck.
The fatal park attack happened following a row over a bottle of Buckfast.
A fourth Malavin brother, Kris, was jailed for seven years in 2014 after he tried to run down and kill a key witness in the murder trial.
A convicted killer has been sent back to jail after he admitted carrying out a brutal attack on Christmas Day, months after being freed early.