Not much is known about the Demon Swords except that they have a strong curse placed on them which brings misfortune to their wielders. All five 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 Sword of the Sun, is the strongest Demon Sword. Known as the Demonic Emperor Sword (魔帝剣 Mateiken), it is a Dragon Slayer sword with the destructive power of the Holy Sword Durandal, which causes Yuuto to compare it to a combination of Ascalon and Durandal, but does not need to be charged to do destructive attacks, which Yuuto thought meant it was "more" powerful than Durandal and is sharper than the Holy Demonic Sword he creates. It was previously used by the original Siegfried to slay the Dragon King Fafnir (though Fafnir was later revived by the Norse Gods).
Balmung (バルムンク Barumunku) is a Demon Sword from Norse mythology with a drill-like aura. It is capable of creating powerful whirlwinds.
Nothung (ノートゥング Nōtungu) is a Demon Sword that can rip through space-time.
Tyrfing (ディルヴィング Diruvingu) is a Demon Sword that can create craters.
Dáinsleif (ダインスレイヴ Dainsureivu) is a Demon Sword that creates huge pillars of ice from the ground.
- 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 Svarflami.
- Gram is currently the only known Demon Sword that possesses two abilities: Dragon-slaying abilities and destruction abilities.