Tower Dungeon 5 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.

Having joined forces, unit seven of the Royal Guard and the newly arrived members of the Anti-Nausea Alliance forge ahead through the Dragon Tower. Though it was already crawling with monsters, the ancient monolith somehow continues to become more and more dangerous thanks to the Necromancer’s undead thralls—but this intrepid band of nine warriors and one crow-goat fight on nonetheless. And, in the midst of desperate battle, the shocking tale of Aridelia’s past is finally revealed…!