Thanatos

Thanatos (Θάνατος in Greek) is the god of death. He is a son of Nyx and the twin brother of Hypnos. His Roman counterpart is called 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 Son Of Neptune
Thanatos acts as a lieutenant of Pluto (Hades) and acts as border control to keep souls from crossing between life and death without his permission. The new Thanatos picture shows him in chains. A theory is that Gaea's forces chained him up so she could control the doors of death.[1] 

Appearance
Thanatos is a good looking black guy who has light brown eyes and plaited hair. He is very muscular and has two huge wings with black and purple feathers.

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

Trivia

 * Thanatos the name means death in Ancient Greek just as Mors means death in Latin.
 * Thanatos the word is a term for the killing instinct.
 * He is covered in chains. Hera/Juno also said that death must be unleashed before feast of fortune to the campers of Camp Jupiter.