Orient 15

The reassembled Obsidian Eight have launched an all-out assault on the Land of the Setting Sun. After speaking to Lord Hojo, Musashi and his friends learn that their best hope may lie in special black blades forged from the body of a demon god. Musashi and Kojiro then set off on a perilous journey into the bowels of the earth itself in search of the precious dark metal.

Volume 15 of the Orient manga series by Shinobu Ohtaka.