QUESTION: Is there any other sin besides the five rebellious sins that causes us to fall into the Hell of Incessant Suffering?
ANSWER: Yes, there is. The grave sin of slandering the True Dharma.
QUESTION: Is there any scriptural proof for this?
ANSWER: The Lotus Sūtra, fascicle 2 (chapter 3 on “A Parable”), preaches, “Those who do not believe in this sūtra but slander it, … will fall into the Avīci Hell when their present lives end.” It is clearly stated in this sūtra that committing the grave sin of slandering the True Dharma is the cause of falling into the Avīci Hell.
Ken Hōbō-shō, A Clarificaton of Slandering the True Dharma, Writings of Nichiren Shōnin, Doctrine 3, Pages 113.