Decarabia is a long, fish-like powerful Lord who serves as Supreme Commander for the various armies engaged in the war of the Snake of Festival. He has a useful Method of Unrestraint, Providence, which allow him to detach his scales to be given to the Denizen Armies. These scales act as a medium for him to perform Powers of Unrestraint, and also as a form of communication. He can also use these scales as a medium of access for other Crimson Denizens to cast their spells. His signature move is "The Scale of the Resonant Depths", which he used to disperse the Flame Haze armies under Demantius. He oversees the operations commencing in the battle. (Wikia)

Continuing from the events of the last series (Shakugan no Shana II), both Shana and Kazumi stand at their respective locations waiting for Yuji to meet with one of them, only to discover that Yuji has disappeared, with not even the slightest evidence of his existence left behind. All is not lost however, as the letters the two of them sent to him still give hope of Yuji's continued existence. As Shana and company search for answers on Yuji's whereabouts, they soon find the truth staring back at them when Yuji reappears in front of them as the leader of Bal Masqu&eacute;. Left with little choice, Shana must now confront her most unlikely adversary on the battlefield in her toughest challenge yet.
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(Source: Anime News Network)