Stolas is a Great War veteran and an affiliate of Bal Masqué. 

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)

Invaders from the Crimson Realm have descended upon Earth looking for something called the Power of Existence. Whoever possesses this power will control the very fabric of existence. To the rescue comes an otherworldly young girl with fire in her eyes. She may be cute and petite, but she's endowed with enough preternatural strength to save the entire universe. Plus, she's got a really big sword!
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(Source: Viz Media)