Tower Dungeon 2 by Tsutomu Nihei

Tsutomu Nihei, master of the mysterious architectural monolith, brings his genius to bear on the fantasy genre for the first time. Having established himself with hit series like Blame! and Knights of Sidonia, he has now created a gritty, dark fantasy world and populated it with evil sorcerors and unique undead horrors.
The ingenuous farmboy Yuva must accompany a contingent of knights through the increasingly dangerous floors of massive tower on the way to rescue the princess, Nihei bringing his unique sensibility to bear on a crumbling world of malformed creatures, wounded soldiers, and labyrinthine darkness.

Eriquo, Lilicen, and Yuva are left to rescue the princess on their own after their commanding officer, who just so happens to be the king's sister, is whisked back to the capital to sit the throne in his absence. The three face innumerable dangers as they begin to tease out the secrets of the Dragon Tower—but danger lurks without as well as within. Haughty knights, mighty sorcerers, a killer with the indomitable power of the Dracomorph… Monsters come in many forms!

This dark dungeon-crawling fantasy from master of science fiction Tsutomu Nihei (Blame!, Knights of Sidonia) continues to expand its scope, hinting at a wider narrative set in what is surely his richest world yet.

Do you have what it takes to face the perils of the Dragon Tower…?