By the Grace of the Gods: Volume 14 (Light Novel)

Ryoma is hard at work grinding his adventurer rank in preparation for his journey into the Sea of Trees of Syrus. En route, he and his companions decide to clear out the Undead-ridden City of Lost Souls, where Ryoma thinks outside the box to overcome obstacles-like inventing a new spell based on his knowledge of Earth. However, things change when they find a massive pocket of magic crystals buried under the city's tower! As Ryoma mines the crystals, he finds something that will surprise even the gods themselves.