I just got the scan of this month's issue of Monthly Shonen Magazine, featuring interview with Masatoshi Kawahara-sensei and at the end there's a page detailing the story (I might going to translate this later).
The gist is that the will story set in Showa Era, focusing on Tsukumo's mother, Shizuru Mutsu (huh, never knew that Tsukumo's mom a legit Mutsu), Maeda Shingen (the man Tsukumo fought resulting on his trauma), and Utsutsu Fuwa (Yamada.... Oops! Spoiler!). Volume 16 of Dai ni Mon actually teases this arc when Yamada is talking about his past. There's a scene where Maeda made a promise with Shizuru that he might going to kill her son (and Shizuru was like "Oh, sure! Go ahead!"). This is the second arc in Shura no Toki that directly related to the main series (first one being the Western Arc).
What I'm expecting of this arc: a sexy scene. The only sexy scene Masatoshi Kawahara ever drew in this series is during Minamoto Yoshitsune arc . Even that were only side boobs. And the last time he ever drew woman fully naked is Paradise Gakuen, and that was about 20 years ago (Kaiouki's final volume cover barely counts). Man, I'm a pervy bastard!
Also if this sells well, I hope we're getting season 2. I want to see Azuma get animated!!!