Taiyo

Sun is a boy who's lived his whole life underground and never once seen the sun, which changes the moment he wakes up a monster boy with no idea how he got there. He has to find a way out of a forgotten underground world while dodging things that want to eat him, or worse, turn him into their new toy. The sun, for the record, wouldn't hurt an ant. Everything down here would.

The setup is simple horror-eroge logic: explore ruined caves with nothing but a lantern, and if a monster catches you, the game cuts to her claiming you instead of letting you leave.

You walk the caves point-and-click style, holding a lantern against the dark and running when running is smarter than fighting. Get cornered by the wrong girl and Sun stops being an explorer and starts being furniture, with the scene playing out before you're dumped back to try the exit again. There's a save system so you don't have to relearn every mistake from scratch, and an HP counter the game insists you don't need to worry about, which is exactly the kind of thing you should worry about.