Grimoire

Rika breaks into a magical library chasing a specific tome, only to find another thief already cracked it open. The theft doesn't go as planned: she's knocked down into the dungeon underneath the library and has to fight her way back to the surface.

Climbing out means picking up new spells as she goes, since the depths are full of monster girls and tentacled things that don't take kindly to trespassers. Losing to them plays out the way you'd expect from the tags rather than a game-over screen.

It's a side-scrolling platformer at heart: move, jump, dodge, and use whatever spells Rika has scraped together to get past the next monster and the next floor. The setup is simple and the source doesn't dress it up as more than that.