Furies

The Furies (Ἐρινύες in Ancient Greek, Erinyes) are goddesses of vengeance who serve the god Hades. They were his torturers in the Underworld and punished the crimes of the wicked. Their number varies, but most commonly there were said to be three. Each one was sometimes represented as avenging a certain type of crime, such as murder, marital infidelity, or patricide. They are often referred to as the 'Kindly Ones' in order to remain safe from them, however ironic it is.

History
When the Titan Kronos castrated his father, Uranus, and threw what was left into the sea, the Erinyes emerged from the drops of blood, while Aphrodite was born from the sea foam. According to a variant account, they issued from an even more primordial level—from Nyx, 'Night'. For the Orphism are Hades and Persephone the parents of the Erinyes.

History recognized three:
 * Alecto
 * Megaera
 * Tisiphone

In the story of Canto VIII of the Inferno they confront the poets at the gates of the city of Dis. The heads of the Erinyes, whom the two poets met in Canto IV, were wreathed with serpents and their eyes dripped with blood, rendering their appearance rather horrific. Sometimes they had the wings of a bat or bird, and the body of a dog. Since the Greeks were afraid of saying their names they are called by different eufemisms.

Description
The Furies are described as old, but virgin women with black skin. Their hair is made of snakes and they wear grey robes.

Yet, in the books, they have leathery wings like bats, claws, a mouth full of yellow fangs and glowing eyes. They carry whips that dance with flames everytime they flick them.

The Lightning Thief
Alecto is disguised as Mrs. Dodds: a pre-algebra teacher at Yancy Academy. She dislikes Percy Jackson and always seems to prefer Nancy Bobofit, the class bully. It is later discovered that she was actually one of the Furies and thus not a human. On a field trip to the Metropolitan Museum of Art, Percy kills her with Riptide (Although he does not realize it is Riptide yet; he thinks that he hallucinated everything that happened, including Mr. Brunner throwing him the pen.) and the monster disintegrates into dust.

Percy later fights Alecto (along with the other two Furies) again while on a bus just as he is setting out on his quest, and wins with the help of Grover and Annabeth.

Percy encounters the Furies when in the Underworld upon encountering Hades. After hearing what really occurred with the theft of Zeus' master bolt and Hades' helm of darkness during Percy's fight with Ares, the Furies arrive once the battle has ended, and Percy gives them the helm to return to Hades.

The Sword of Hades
Nico summons the three furies to take himself, Bob (Iapetus), Percy, and Thalia back to Hades' Palace.

The Last Olympian
It is revealed that Alecto was the "lawyer" who brought Nico and Bianca di Angelo to the Lotus Hotel and Casino under Hades's orders. After about 70 years, Hades had the Fury get his children out as he hoped that one of them could be the child in the Great Prophecy. The Furies also appeared to carry Nico and Percy to Hades's throne room before Percy bathes in the River Styx.

Trivia

 * Alecto seems to be the leader of the three because she is always in the middle and Hades always makes her do the work.