By The Grace Of Gods Light Novel Sc Vol 14 (C: 0-1-2)

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…