I'm about to go to bed, so I won't be able to follow up until the next morning so I'll try to put down all the details I can remember. The protagonist was a child whose village was attacked by demons, she flees into the mountains or forest; can't remember which; and ends up inside a secret dungeon. The dungeon is extremely dangerous, but she is able to advance through it slowly, leveling up as she goes. This is all backstory, the story begins with her fighting against the dungeon's final boss, and defeating it. She then escapes the dungeon and goes to a nearby village. At this point we learn that a many years have passed since she first entered the dungeon, so the protagonist is now an adult or teenager; again can't remember. She follows the traditional route registering as an adventurer, though I believe she had an overarching goal of finding out what happened to her village. I think there was some sort of conspiracy, but I do know demons were definitely involved. I'll put any other bits and pieces I can remember in the tags.
ooh the old one as always, an interesting novel will leave its mark kansutoppu a.k.a counter stop a good read