By the Grace of the Gods: Volume 14 (novel)

To prepare for the journey into the Sea of Trees of Syrus, Ryoma's hard at work grinding his adventurer rank. He and his companions decide to clear out the Undead-ridden City of Lost Souls on the way, and thanks to Ryoma's knowledge of Earth, he manages to invent a new spell. The group also makes an unexpected discovery under the city's tower--a massive pocket of magic crystals! Ryoma starts mining them, but he finds something else that would surprise even the gods themselves...