Melinoe

Melinoe is the goddess of ghosts, sometimes confused with Hecate.

The Demigod Files
Melinoe shows Ethan Nakamura the way out of the Underworld, and takes on the appearance of Thalia and Nico's mothers to confuse and torment them, but cannot fool Percy, who breaks the illusion. He swings at her with his sword, and she dodges, only to be hit by Thalia with an arrow. She then vanishes into the fog.

Appearance
Melinoe is a strange goddess, and her appearance is frightening. Her left side is black and hardened like a mummy, and her right side is pale and chalky as if she were drained of all her blood. She wears a golden dress and a golden shawl, and her eyes are empty black voids.