The Harpē (ἅρπη) was a type of Sword or Sickle; a sword with a sickle protrusion along one edge near the tip of the blade. The Harpe is mentioned in Greek and Roman sources, and almost always is mythological contexts.
The Harpe sword is most notably identified as the weapon used by Cronus to castrate his father, Uranus. Alternately, said weapon is identified as a more traditional Sickle or Scythe. The Harpe, Scythe or Sickle was either a Flint or Adamantine (Diamond) blade, and was provided to a then-unborn Cronus by his mother, Gaia:
While Uranus kept siring children with Gaia, he would not let her give birth to them, for fear of being overthrown by his own children. This state of affairs left Gaia in incresingly excruciating pain, as she fell pregnant with even more and more children, all of who she was prevented from birthing. Gaia asked each of her unborn children to rise up against Uranus and free her, but was refused by all but the youngest, Cronus. So, Gaia provides him with a blade, (a Harpe, Sickle or Scythe); and when Uranus next came to lay with Gaia, Cronus lept up into action and castrated his father, overthrowing him and driving him away forever. Thus the blade, (either a Harpe, Sickle or Scythe), became a symbol of Cronus' power.
Perseus, (a demigodgrandson of Cronus'), is also regularly depicted in statues and sculpture, armed with a Harpe sword in his quest to slay Medusa and recover her head. Perseus was provided with such a sword by his father, Zeus (Cronus' youngest son and later overthrower).