A son, who had only recently reconnected with his real dad, left him fighting for life in intensive care after giving him a savage beating.
Paul Knight, 30, had grown up thinking his stepfather was his dad and was shocked to find out the truth when he saw his birth certificate, aged 17.
Years later, he finally met his biological dad, Paul Smith, and the pair began to socialise occasionally.
But on one such night out, trouble flared and when the pair got back to Mr Smith's flat, and things turned violent, Chronicle Live reports.
It is not clear exactly what happened but Mr Smith ended up with serious injuries, including multiple fractures to his ribs and vertebrae and eye socket.
Jailing Knight at Newcastle Crown Court, Recorder Carl Gumsley told him: "It may never be clear exactly what happened that night.
"What is known is by the end of that night you left him seriously injured, no doubt in a lot of pain and distress and you just left him there in a pool of blood and there were spatters of blood.
"What clearly took place was a vicious and brutal attack.
"Whatever the circumstances and whatever the provocation - you say it was self-defence gone too far but I can't help but note you were noted to have no real injuries.
"He had broken ribs, a broken eye socket, bleeding in his abdomen, fractures to his back and he spent six days in intensive care."
The judge said Knight was already subject to a community order at the time and was waiting to do an anger management course when he attacked his dad.
Knight had resumed contact with Mr Smith in 2016.
They went out together in July 2018 and ended up at Mr Smith's flat at the end of the night.
After subjecting him to a sustained attack, Knight left, locking the door and posting the keys through the letter box.
The next morning, Mr Smith's daughter turned up and he managed to crawl to the door and was taken to hospital.
The court heard his extensive injuries included more than 10 fractures to his ribs, fractures to his vertebrae, to his eye socket, bleeding in both eyes, swelling to his nose and jaw and wounds to his face.
Prosecutor Anthony Moore, referring to a victim impact statement, said: "He describes afterwards he felt vulnerable, it's changed him, he doesn't feel safe going out, he worries for his and his partner's safety.
"He said he lost two stone due to worry and injuries."
When Knight, of Cairo Street, Hendon, Sunderland, was interviewed by police, he claimed his dad had got in trouble while they were out and when they got to his flat he headbutted him and he "reacted in self-defence".
Knight, who has 20 previous convictions, pleaded guilty to inflicting GBH and was jailed for two years, and give a three-year restraining order forbidding him from contacting his dad.
Tony Hawks, defending, said: "The background with his father is highly unusual. Until the age of 17 he believed a man who was his stepfather was his real father.
"By accident, when an adolescent, he came across his birth certificate and realised that was not the case and it was, in fact, the complainant who was his real father. That was a serious psychological shock.
"He made contact with his father in June 2016 and they saw each other intermittently. The defendant had an ambivalent attitude to him for what might be considered understandable reasons."
Mr Hawks said Knight was annoyed with his dad for allegedly causing their night out to end on a sour note and said they fought and he "lost control".
He added: "He is properly remorseful.
"However it started, he behaved abominably."
Mr Hawks added that Knight has been diagnosed with depression, anxiety, and epilepsy.
