Based on a Japanese light novel side story series, written by Fujino Omori and illustrated by Kiyotaka Haimura (based on the designs by Suzuhito Yasuda).
Dungeon ni Deai wo Motomeru no wa Machigatteiru Darou ka Gaiden: Sword Oratoria adapts the first 4 volumes of Fujino Omori's light novel series of the same title.