Ummm. It's been awhile for me. If we are talking about the horrendous part
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1. MC has a senpai and this senpai later would uh, gangraped by the mermans who want to mate with him. There are no explicit depiction of it the deed, but there are some implication for the aftermath.
2. There is this system Agares think of, how he wants the merman mates with the soldier (cause it's makes them stronger or smth, i forgot) so he let merman kidnapped them. These soldiers later put on some pods or smth then MC released them. But because he released them. They began a mass suicide falling down from a cliff to the sea. Like, a lots of people falling down to the reefs.
So yeah, there's the one that I remember. The story kinda too much and full of violence and gory details sometimes.
i got spoiled and they say is so gruesome huhu