I saw the raw too, there was never anything that gruesome in the WN, closest was a r*pe scene (completely unrelated girl, a few books before this dungeon) that's pretty tame in comparison.
In the web novel, it was indeed a team of adventurers behind the abnormality, they were the final fight. But there, this team just split up after hitting on the girls, and got bribed for first crack at the dungeon boss.