The Graiai are probably best known for the role they play in Perseus's quest to slay the Gorgon and bring it's head back to King Polydectes
The Graiai are three sisters, also known as the Grey Sisters or the daughters of Phorcys, and are depicted as three old hags, who have only one eye and one tooth to share between them. When Perseus confronts them on his quest they must pass the eye around between them to converse with him, often fighting between themselves.
There names are Deino, Enyo, and Pemphredo. Their parents were Ceto and Phorcys and they were sisters to the Gorgons.
As with all Greek Mythology there are various versions of their encounter with Perseus. The most popular being that while the sisters were arguing Perseus stole their eye and forced them to reveal the location of the Gorgons.
Phorcys, their father, was a sea god and most stories describe the Graiai as personifications of white sea foam. They have also been described as having the bodies of swans.