I think you're both asking for the same thing. IIRC my lore correctly, Witcher clans were founded after the Conjunction to deal with the sudden influx of monsters. A game about the first Witcher could be set mere months after the event.

[–] FeelzGoodMan420@eviltoast.org 2 points 4 days ago* (last edited 4 days ago)

I sound like a huge nerd here but i believe witchers were created 500 years prior when the humans first got to the continent and encountered monsters. Both humans and monsters came from the conjunction. It is never explained what humans were doing for the first 1000 years after the conjunction (conjunction was 1500 years ago) prior to arriving on the continent. I assume they also had to deal with monsters or maybe the monsters were dropped only on the continent? No idea. The author was unfortunately not very consistent or clear on historic lore. Perhaps it was intended. I'm not sure.

Source: I spent way too much time looking at lore explanations and engaging on forums.