Thanatos

Thanatos, (Θάνατος in Greek) is the minor god of death. He is a son of Nyx, and the twin brother of Hypnos. His Roman counterpart is Letus or Mors.

History
Thanatos was called the god of peaceful death while the Keres, were his antithesis as the spirits of violent death. Once he was tricked by Sisyphus which was one of the reasons why the mortal was condemned to the Fields of Punishment. When Thanatos came to take Sisyphus to the Underworld, he chained the god to a rock and effectively stopped death until Ares, freed him.

The Demigod Files
In The Sword of Hades, Nico mentioned Thanatos when he tells Thalia and Percy about Sisyphus.

The Lost Hero
In the "Gods and Goddesses in The Lost Hero ", Thanatos is mentioned as a brother of Hypnos.

The Son Of Neptune
It describes Thanatos as being good-looking and the controller of where souls go after death. Thanatos serves as Pluto's lieutenant, preventing souls from crossing between the lands of the dead and the living.

In the book, he has been captured and is being held in Alaska. Percy, Frank and Hazel go on a quest to free him, and suceed.

Appearance
Thanatos is lean and muscular, with a regal face and black hair flowing down his shoulders. His wings glimmer in shades of blue, black, and purple. He is frequently mistaken for the god of love. Hazel Levesque describes him as the most beautiful man she has ever seen.

Personality
Thanatos was believed to be merciless and undiscriminating, sharing a mutual hatred for both the other gods as well as mortals.

Trivia

 * Thanatos the name means death in Ancient Greek just as Letus means death in Latin.
 * Thanatos the word is a term for the killing instinct.