Duncomb, who goes by the name Black Rebel, claimed he was doxxed by Antifa militants, who were tracking his movements on social media, telling others to "watch out" for him.
According to court documents obtained by the Oregonian, he claimed a group began tailing him and his friends; he handed his camera to one of the friends and decided to confront one of the men following him.
The shocking video shows the assailant walking behind the group, his face completely obscured by a helmet, ski goggles and mask, as the filmer acknowledges "someone's stalking us."
Duncomb, who had circled back behind him, suddenly comes up from behind and wraps an arm around his shoulder, asking: "Hey buddy, why are you following us?"
Without a word, the man pulls his hands from his pockets and jams something into Duncomb's back, who reels away crying out in pain.
"Did you just stab him? Bro he just stabbed him dude!" the friend calls out. "The guy right there! Get him! Get him!"
The assailant flees, but doesn't get very far. Another frightening video shot on the night shows the crowd swarming on him, punching, kicking and stamping on him.
He is dragged by the scruff of the neck behind police lines and into custody.
Police arrested Blake David Hampe, 43, at the scene. He faces second degree felony assault charges, and unlawful use of a weapon. His bail is set at $250,000.
According to his probable cause affidavit, he told officers Duncomb had "set up" on him and tried to choke him.
"I was stabbed for being a conservative journalist," Duncomb claimed afterward.
"That knife was long enough. He stabbed me to kill me," he said in a Periscope livestream from his hospital bed. "We were all out there, four friends, trying to keep each other safe in the best way possible. I figured I'd try to de-escalate and just have a conversation.
"I went over there, said 'what's up buddy?' and that motherf--ker went around and shanked me in the f--king kidney."
Duncomb said he himself also had a knife and a can of bear mace at the time, "but I didn't want to endanger innocent bystanders."
He said his main purpose for being in Portland was to show "that it wasn't just the Feds creating the problems."
Court records show that in 2008, Blake David Hampe was sentenced to 41 months in prison and ten years of supervised release on child pornography charges.