When a nameless protagonist awakens as a dungeon, he's got to figure out everything from scratch. But while digging rooms, playing with mana, and unlocking new abilities is fun, his decision to save the life of a fox-kin who seeks shelter drastically alters both of their lives. Because Shayma is on a mission for her Queen, to gather the parts of an ancient relic, to defend their kingdom from the mage-kings and their captive dungeons . . .
So I'll get the warning out of the way first. This book has very explicit sex scenes. However, the chapters in which these occur are clearly marked, and can be skipped once those scenes start with zero loss of plot. I very much appreciate this, as it makes the book work like a fade-to-black scene if you just move on, thus allowing the reader more control over the level of graphicness present.
Luckily, there's plenty else to like. I'm particularly fond of how Blue (named for the color of his dungeon core crystal, since he can't remember any other name) is trying his hardest to figure out what is this magic stuff, and why doesn't it make logical sense. He's got some nice physics-based tricks he can pull, but magic is cheating, and high leveled Classers can essentially ignore some of the things he would otherwise think could harm them (finding burrowing critters that EAT lava rather than get smoked out by it was hilarious).
But while he's trying to go around crafting stuff, there's a whole other story unfolding outside. The kingdom of Tarnil is under assault, their Queen having lost most of her cities to the invading dungeons, and she has precious few resources remaining. Which is why Shayna decided to take things into her own hands and look for the one weapon that might turn the tide. Which leads her to Blue. And now Blue is involved in that war, far more than he'd like to be---because the mage-kings definitely won't let a "free" dungeon like him alone, so it's in his best interest to cooperate at least to some extent in the fight against them.
There's also the problem of Depletion, and the wonky rules of Powers, and so many more fascinating bits of worldbuilding going on. And the dragon Blue finds that also lives on the mountain he has been claiming.
Overall this is one of the best dungeon core books I've read. With its heavy focus on characters as well as worldbuilding, the abundance of cool stuff Blue creates, and a mana economy (in-dungeon) based on flowers and environments, of all things, the parts all come together beautifully. I rate this book Highly Recommended.