The Unwanted Undead Adventurer (Manga) Volume 12 by Yu Okano

Having overcome his former masters’ trial, Rentt earns knowledge of his hometown’s ancient secret, and its dizzying scope is beyond anything he could have imagined; a bloody history, a forgotten dungeon, and long-lost magic are just the beginning.

While he and Lorraine come to grips with their discovery, the Gold Class adventurer Nive and her compatriots draw closer to rooting out the vampires planning to attack Maalt. They'll have to move fast to prevent tragedy from striking—but little do they know, it may already be too late...