Shakugan no Shana X: Eternal Song is based on the events of the tenth Shana novel.

The events in Eternal Song focus 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.

(Source: Shakugan no Shana Wikia)