Dragon King Sword
The Dragon King Sword (竜王剣, Ryuu Ou Tsurugi) is a famous sword and heirloom of the Scarlet Moon Empire.Scarlet Moon Empire, the Dragon King Sword is apparently a form that the Sovereign Rune has taken. As a result, wielding the sword meant that Emperor Barbarosa was immune to the effects of Windy's Conqueror Rune throughout the Gate Rune War.
With the Sovereign Rune, the Dragon King Sword could transform its wielder into a three-headed dragon. The location of the sword after Barbarosa's defeat is unknown.