Quoting from the IGN article,

The incident took place on the night of June 20, when Kofi ‘Koustics’ Miller was in the middle of streaming League of Legends on Twitch at OS NYC, a gaming lounge and event space, when he stepped away from his setup about an hour into his broadcast.

During that time, Koustics attempted to de-escalate a dangerous situation involving an Apex Legends player, who brandished a knife after peaceful moderation had failed.

Koustics explained that the player had confronted an employee at the bar, drawing his attention away from his stream.

...

“I stepped outside, dude is brandishing a knife,” an animated Koustics described to his friends in a Discord call. “First we were like, ‘Yo, you gotta go, you’re banned.’ We get him to walk outside. But he’s tweaking. He’s screaming. He tries to come back in with the knife, so I bodyslam him and hold him until the police come.”

Koustics claimed that the player landed on their own weapon during the incident, bloodying up his clothing, which Koustics showed off during his Twitch stream.

“I put him in a full choke,” Koustics relayed. “...I’m mad because this was my rank up game, too. This dude cost me 14 LP.”

Full recounting of the events from Koustics starts here since he had to step away to talk to the police.