Furies

The Furies (Ἐρινύες in Greek, Erinyes) are goddesses of vengeance who serve 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 'Kindly Ones' in order to remain safe from them, however ironic it is.

History
When the mighty 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 seafoam. 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.

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 mouthful of yellow fangs and glowing eyes.

The Lightning Thief
In The Lightning Thief, Alecto is disguised as "Mrs. Dodds", a pre-algebra teacher at Yancy Academy. She disliked Percy Jackson and always seemed to prefer Nancy Bobofit, the class bully. It was later discovered that she was actually one of the Furies and not really human at all. 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 just thinks that he's having a hallucination after Mr. Brunner throws him his pen) and she disintegrates into dust.

He later fights Alecto again, while on a bus just as he is setting out on his quest, along with the other two Furies.

Percy encountered 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 arrived after the battle and Percy gave them the Helm of Darkness to return to Hades.

The Demigod Files
In The Sword of Hades, the three furies are summoned by Nico to take Bob (Iapetus), Percy, himself and Thalia back to Hades' palace.

The Last Olympian
In The Last Olympian, it is revealed that Alecto was the "lawyer" who brought Nico di Angelo 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.