Manga author Yoshiaki Sukeno has announced that the third novel in the Twin Star Exorcists will be launching on December 4. Shueisha will publish it and Hajime Tanaka is coming back as the writer. The focus of the novel will be Seigen, Shimon and Tenma's mysterious past. You can find some rough sketches below.
The first novel in the series came out on August 4, 2016 and is written by Hajime Tanaka and drawn by Yoshiaki Sukeno. The manga series with the same name is written and drawn by Yoshiaki Sukeno. It has been publishing since November 2, 2013 and is serialized by Jump SQ. The anime series aired from April 2016 to March 2017 and has 50 episodes in total. Crunchyroll has the English license and Studio Pierrot animated it.