Seth Black's history is very scare, vague and purposely hidden.
What can be said is he was found in a major bust, where illegal guns were trafficked into Chernarus. Those receiving such were the Red Star, where Seth was convicted of profiteering off of the suffering of those during the war. Yet his place in all of this remained very unknown, for they couldn't identify him in anyway. It was clear he wasn't from Chernarus, yet they couldn't get his nationality, finger prints, his face didn't show up in any police profiles, nothing. Despite harsh questioning and the occasional violent persuasion, he'd almost mock every interrogation with his story continuing to change, wasting vast amounts of police time. Where he was from, what he was doing, what his role was... everything. The only thing that stayed consistent was his name.
His behaviour led to him being incarcerated with mental health, as every lie detector test came back positive for the truth, meaning that every lie he told, was said with truth, leading to him being believed as mad. Rather than have the state deal with him he was simply committed to St Alban's Asylum, a massive asylum which housed any form of mental heath from paranoid schizophrenics to the criminally insane.
When the Outbreak hit, Seth, along with many many others were in the middle of a months long planned escape, having chose either the worst or the best time to do this, for The Asylum, on fire, was assaulted by the infected, the police and with the freed patients, chaos ensued... but that story is on need to know. (Future group plan)
All that is known is that when he made his escape, it was under a mass panic where inmates fought the police, police fought the infected and the infected fought the inmates as St Alban's burned to the ground. Seth fled into the woods with Phelan Sokol, using the chaos to escape the madness which ended that part of his life.