?

Raping him.

She raped him. Though, I guess, it's quite disturbing to show explicitly. Hence, we only see her taking off her underwear,and

Honestly, I think Koogs just did it to show hints but not lwt out the full story to keep the mystery element. Since the rest of the entire manhwa sees disturbing as an understatement

Yea, but a rape scene featuring elements of incest is heavier than the "normal" disturbing things we are used to. However, I can see your point about the mystery factor. It can even leave some room for a different interpretation to what took place in this chapter, as none of our pov characters are considered reliable narrators.