Brynhildr is a demonic cursed sword and it is used by the ex-hero Basara Toujou. As stated by Chisato in the 4th Volume of the Light Novel and in the third Episode of the Second Season it's spirit was that of a strong and feared Valkyrie which used to collect souls for the Chief God, as seen in the Anime when his powers are released the only way Chisato was able to calm the enraged Basara after Bryhildlr took control of his body was to give him the soul of Ornis and let Basara extract all of that rage into the illusion which Chisato made for him.