Jack Woodley was killed after he left Houghton Feast last October, by a group of teens who were "out looking for serious trouble" and had pulled on face coverings and balaclavas before they pounced.
Newcastle Crown Court heard one of the attackers was heard to shout "get the chopper" out as they approached the 18-year-old victim, with one of them carrying a Rambo-style knife.
Jack was then punched, kicked, stamped on and stabbed in the back, while the group "prevented him from escaping and prevented other people from helping".
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One eyewitness said the youths were like a "herd of lions" when they attacked.
Another witness who saw the violence said: "Jack got down on the floor so he could curl up in a ball.
"He was so scared. There were so many punches raining down on him. They were brutal.
"Everyone in the group was just kicking the life out of Jack, stamping on his head.
"The lads were savagely kicking Jack, like zombies attacking an animal.
"I remember the terrified look on Jack's face as he was being kicked, punched and stamped on."
Jack, who had told his attackers he wanted to go home and did not want to fight, died in hospital.
Ten youths, now aged between 14 and 18, denied murder and have been tried by a jury at Newcastle Crown Court.
After a trial lasting two months and three days of deliberations the jury has now found all ten guilty of murder.
They will be sentenced at a later date and have all been remanded in custody, facing life sentences.
Prosecutor Mark McKone QC told the court Jack and the youths who killed him had all been at the funfair that evening and the killing happened near the Britannia Inn pub in Houghton.
Mr McKone said it was a 16-year-old defendant who attacked Jack first and the "others joined in".
He said: "Jack was punched and kicked and stamped on and stabbed in the back with a large knife.
"He died the next day after valiant attempts at the scene and in hospital to save Jack's life."
Mr McKone said one of the youths, who was 15, was armed with the "very dangerous knife" that was used in the stabbing and another 15-year-old was carrying a knuckle duster.
He told jurors: "The prosecution say, based on the available evidence, the defendants went out looking for serious trouble that day.
"The prosecution say these defendants surrounded and isolated Jack, pulling up their collars and covering their faces with hats and balaclavas as they prepared to attack Jack.
"Someone actually shouted 'get the chopper out'."
Mr McKone said the group "not only attacked him but prevented him from escaping and prevented other people from helping him".
He added: "By the end of the attack Jack had been fatally stabbed.
"He also suffered significant bruising to his head, neck and body."
The court heard the knife used in the attack was a "Rambo-style" knife with a 25.4cm blade.
Mr McKone said that while only one youth inflicted the fatal stab wound, the other nine are guilty due to "the concept of joint enterprise".
He added: "What each defendant did to Jack varies, some used more serious violence than others.
"But the prosecution say this was classic joint enterprise, where a group acted together for the common purpose of causing really serious injury to Jack.
"The prosecution say all 10 defendants participated in this joint attack, using violence themselves or intentionally encouraging others to use violence.
"To be guilty of murder, a defendant must intent that a victim is killed or intend that a victim is caused really serious harm.
"It follows, that a person can be guilty of murder without intending for someone to be killed."