I still haven’t watched any of the anime, but I do believe there was origin prior to the party. However, it was peppered throughout the manga and not part of the first chapter. Though, most of the “prior to getting to the dungeon” that we see in the manga is similarly peppered iirc.

Tbh it’s one of the reasons I didn’t think this manga would work well as an anime without some rework. It’s story is oddly structured, and it was a little lacking in the story department beyond

Maybe spoilers

I’m super powerful now, let’s literally torture the people that ruined my life.