Underdark: Prisoner Monster Girl Isekai

An archaeologist's expedition ends with him falling through an ancient portal into an underground world where humans are rare, valuable, and hunted. No weapons, no allies, no way back that he knows of, just demons, old magic, and creatures that look at him like a meal or a prize.

The setup is survival dressed as captivity. He's not exploring this world so much as being absorbed by it, and the story leans into the idea that the longer he stays, the less certain it becomes he'd even want to leave. That slow change in him is the actual hook, not just the danger around him.

Play follows a visual novel structure: read the situation, make choices about how the protagonist deals with the monster girls and factions holding power over him, and watch the underground world's politics of dominance play out through 3DCG scenes. It's built for mobile as much as desktop, so sessions are meant to be short and readable rather than sprawling.