Seven Mortal Sins x Seven Heavenly Virtues Gaiden Novel: The Idle Vassal Demon Lord (七つの大罪×七つの美徳 外伝ノベル 旗本退屈魔王) is a web novel by Eiichi Shitara cross-featuring Seven Mortal Sins's Belphegor and Seven Heavenly Virtues's Raphael. The novel was published on Hobby Japan's "Read! HJ Bunko" novel site from July 27 to December 7, 2018.