Immortal Slaying Scythe, Harpe (不死身殺しの鎌・ハルペー, Fujimi Koroshi no Kama - Harupē?) is a divine sword from Greek Mythology used by Perseus to kill the Gorgon Medusa and one of the many weapons inside Gilgamesh's Gate of Babylon. It is described as a monster-killing holy sword with a special shape similar to a scythe, having the cutting edge on the inside of the blade, though it isn't actually an outstanding sword by itself. Its true strength is its trait of Refraction of Longevity (屈折延命, Kussetsu Enmei?), which is a divine ability that nullifies the "undying attribute" of immortals and inflicts wounds that cannot be restored by any methods other than healing in accordance to the natural laws. It is one of the five Noble Phantasms given to Perseus, the greatest amount of legendary artifacts given to a single person in Greek history, for his quest to slay Medusa. It was given by Hermes, and as it was returned to him once the task was completed, it can be called an “Anti-Medusa” weapon.
Medusa in her child form as a Lancer wields Harpe as her primary weapon for combat. With it, Ana was capable of stripping Gorgon of her immortality in Babylonia Singularity. Her scythe contains a long chain for which she can launch the blade head with, or fight and ensnare with the chains themselves, and even as a rope to swing around the battlefield. Medusa Alter also wields the scythe Harpe as her primary weapon. However, hers is similar to Perseus' in appearance.
Harpe was an Authority of Zeus which he used in his battle with Typhon. Typhon stole it from him. Bai Ruolong, a magus of the Wandering Sea fed the flesh of Typhon and in possession of his Dragon Core, should possess the capacity to utilize Harpe, but makes no attempt to utilize it in his second battle with Ergo even though it could have changed the outcome of the fight in his favor. Lord El-Melloi II's analysis of the battle causes him to theorize Bai Ruolong may not have access to all of Typhon's Authorities, the cost of deploying Harpe would have taken too much energy to utilize, or Bai Ruolong personally did not want to use it in that fight.