Decent into Avernus IS finished before the start of this game, even if it's not a direct prequel. In game you learn that the tieflings are refugees from elturel, that the other residents drove them out after the city returned calling them devils.
That sums up everything.
That makes sense, but I still think Zevlor's line about filling the night sky with faerie fire (sic) being a custom they picked up in Avernus is odd.
