Shakugan no Shana X Eternal song -Harukanaru Uta- is based on the events of the tenth Shana novel. 
The events in Eternal Song focuses on the events of the Great War, with a certain focus on the lives of two Flame Haze, the previous "Flame-Haired Burning-Eyed Hunter" Mathilde Saint-Omer and "Manipulator of Objects" Wilhelmina Carmel. They were famous as the counterpart of the Crimson Lord duo called as "Pair of Wings" ("Rainbow Wings" Merihim and "Iron-clad Dragon" Illuyanka). Their adventure starts in the middle of the Great War, an event of great turmoil between Crimson Denizens and Flame Haze and a period of great activity between the two factions.

Yuji Sakai, a high school student, is told by strange girl that he died some time ago, and that he is a "Torch", a temporary replacement for erased humans. Torches take on the forms of those erased persons, but after a period of time, they will vanish from existence and the memories of the living. Unfazed by his apparent death, Yuji befriends this girl and names her "Shana" after her sword. Shana resolves to protect Yuji from the Denizens, who would use his special replenishing Torch to disrupt the balance of the world, and Yuji decides to join Shana in her fight.
 
(Source: Anime News Network)
 
Note: Chapter count includes two "extra" chapters.

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!
 
(Source: Viz Media)

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