Reborn as a monster in a dungeon. Spoiler.

[Deeboo]

The book starts with the mc wakng up in the heart of the dungeon as a monster, and at the end you find out the the whole dungeon was a monster and the mc's safe room was its heart.

[Kris]

A guess - Dungeon Heart: The Singing Mountain by David Sanchez-Ponton?

Google search of Goodreads:
site:goodreads.com "fantasy" "dungeon" "monster" "Heart"

[Kris]

Deeboo, what's the genre (which we need to add to the topic header) - e.g., for adults or teens? fantasy, horror, etc.?

Around what year did you read this book?