Guren no Sōyoku (紅蓮の双翼, Wings of Crimson) is an Unrestricted Spell of "Flame-Haired Burning-Eyed Hunter" Shana which forms wings of flames on her back. It allows her flight and aerial combat capabilities. She acquired it while fighting Margery Daw alongside Yūji Sakai, and it became one of her most frequently used Unrestricted Spells.
Guren no Sōyoku is stated to resemble the Manifestation of Alastor. In the novels, they resemble a jet of flames coming out of Shana's back. In the anime, the wings resemble that of a bird. When flying or undoing the spell, bird feathers could be seen falling from them.
Shana is able to fly using a propulsion of flames which she can control, allowing her to stop quickly using a reverse propulsion and turn quickly using a horizontal propulsion.
The wings are a manifestation of the "visualization of flight" and are composed of flames, so Shana can stay in the air without moving them. However, they can be "exploded to accelerate" and use propulsion, so it can be said that when Shana is moving, accelerating, or changing course, the nature of the wings are real flames which possess heat, and emitting the flames produces kinetic energy. This could be why Shana had thought that Guren no Sōyoku would be erased by the fire-protection barrier of the Treasure Tool Azure, but it is unknown whether the wings can disappear in midair.
The wings were never officially named by Shana, but she recognized it as a part of the power of Shinku to "materialize flames as something other than flames".