"One Reason" (一の理 Hitotsu no ri ) is the twenty-first episode the anime of Shakugan no Shana Final.
Chorde manages to reach the World Egg, and rewrites the Psalm of the Grand Order, instead of destroying the World Egg as expected by Bal Masqué. Enraged, the Snake of the Festival demands an explanation from the Flame Haze on what they have done. Shana and Alastor announce their own plans for the paradise Xanadu, which is to create a law where the Crimson Denizens are not allowed to eat humans anymore. Yūji, and the Snake of the Festival resolve to oppose their interference with all their might, and Yūji orders Sydonay to stop holding back against their opponents.
Sale, and Chiara continue their fight against Mammon, and Dantalion while evading the Seeking Researcher's inventions. The remaining The Four Gods of Earth also continue their battle against their respective opponents despite having achieved their goal, but as Orobas, Leraje, Haborym, and Ribesal will learn, the Four Gods of Earth had another wish than just the law that forbids eating of humans: The wish to kill as many Denizens who have consumed humans as possible to create a taboo against consuming humans for Power of Existence.
Elsewhere, Johann has a talk with Yoshida Kazumi knowing that his life is coming to an end. Because of this, he has formed a plan that will allow him and Pheles to continue on in another form. He also has a message for Wilhelmina Carmel from him and Pheles. Kazumi wakes up in the back of the Hyakki Yakō van, revealing that her conversation with Johann was not held in the real world.
Back at the Hyakki Yakō van speeding through the streets, Para discovers that he is no longer in control after Pirsoyn deploys his unique Unrestricted Spell upon them. They speed up the embankment around the Star of Darkness Castle, and find themselves mid-air just as an army of Servant cannons, and Denizen's open fire upon them. It would seem they have run out of luck, and high above them Yūji, and Shana witness the van exploding...
Characters in Order of Appearance
* denotes if the character debuts in this episode
- Yūji Sakai
- Snake of the Festival
- Kazumi Yoshida
- Khamsin Nbh'w
Powers and Abilities Used
- Kyōko Kametani also worked on the eyecatches for the sixth, twelfth, fourteenth, and eighteenth episodes of Shakugan no Shana Final.
- Mutsuki also illustrated the Niconico end card for the twenty-second episode.
- Near the end of the previous episode, Sydonay watches the Chorde rings hit the World Egg while being restrained by Wilhelmina's ribbons and in his base form. When he views the same event in this episode, he is not restrained and in Manticore form, rushing to Hecate straight afterwards.