I have been comparing the Japanese series to the Dark Horse printing and noticed that "Descent of the Mystic City" doesn't seem to be part of the manga right after "Return of the Seima III" (renamed "Return of the Demon Saint III" in Dark Horse version). Instead it is followed by "A Rest for the Siema" (rest of Volume 6) and a 10 parter called "Eyes of the Undead" and a two parter called "Legend of the True Siema" (Volume 7)
Does anyone know what this is and where this is in the original manga?