Not much is known about the Demon Swords except that they have a strong curse placed on them which brings misfortune to their wielders. Five legendary Demon Swords were wielded by Siegfried of the Hero Faction, until Volume 12 when one of the five Demon Swords, Gram, left Siegfried and chose Yuuto Kiba as its new wielder, allowing Yuuto to defeat Siegfried. Following Siegfried's defeat, the remaining Demon Swords also chose Yuuto as their new wielder.
Demon Swords are the exact opposite of the Holy Swords, in which they use demonic power and are extremely powerful against Angels and Fallen Angels. Just as with the Holy Swords, only a select few can wield Demon Swords and they are known to even shorten the lifespan of its user if overused. Demon Swords appear to have a will of their own, as revealed in Volume 12.
Gram (グラム Guramu) also known as the Demonic Emperor Sword (魔帝剣, Mateiken). It was once wielded by the original Siegfried to kill one of the Dragon Kings, Fafnir before the latter was resurrected by the Norse gods, it was passed down Siegfried, an artificial human who shares both his name and genes, then it chose Yuuto Kiba after turning into a monster.
Balmung (バルムンク, Barumunku) is a Demon Sword from Norse mythology with a drill-like aura, which is capable of creating powerful whirlwinds. It was previously owned by Siegfried, then chose Yuuto Kiba.
- All Demon Swords are named after swords used in Norse mythology.
- Balmung and Nothung are alternate names for Gram.
- Dáinsleif, meaning "Dáinn's legacy" in Old Norse, was a sword wielded by King Högni in Norse mythology.
- Tyrfing is the titular sword used in the Norse legend collection the Tyrfing Cycle. According to the legend, it was a sword forged by the Dwarves Durin and Dvalin. They are forced to do so by Svafrlami, the king of Gardariki, but in revenge they curse the sword so that it will kill a man every time it is unsheathed, cause three evil deeds and be the undoing of Svafrlami.
- Gram is currently the only known Demon Sword that possesses two abilities: Dragon-slaying abilities and destruction abilities.
- Currently all known Demon Swords are not used against Angels, but mainly through Devils, Fallen Angels, Dragons, Monsters, and other demonic beings of darkness.