Perseus' Sword, Harpe was given to him by Zeus along with Hades' Helm of Darkness and winged sandals from Hermes and a polished shield from Athena. Harpe was used by Perseus to decapitate the only mortal Gorgon, Medusa, whose head he kept as a trophy. It shows up several other times in Greek mythology but most famously, it is the scythe created by Gaia and used by Cronus to castrate his father.
In most of its depictions, it has a sickle-like formation on the edgeless side of the sword. In some versions of Perseus' story, it is said to be made of a mythical metal called Adamantine.