Thanatos

Thanatos, (Θάνατος in Greek) is the god of death. He is the son of Nyx and the twin brother of Hypnos. His Roman counterpart is Letus (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. He also wrestled with Hercules for the life of Alcestis, the wife of King Admetus. He lost the match, bringing the queen back to life.

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

The Son Of Neptune
Hazel describes Thanatos as being beautiful and the controller of where souls go after death. Many people mistake Thanatos as Cupid, though he assure's you that he is 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 on Hubbard Glacier. Percy, Frank, and Hazel go on a quest to free him.

The three find Thanatos on Hubbard Glacier in Alaska where he is bound in chains of ice. While Hazel was fighting Alcyoneus and Percy was busy fending off the giant's minions, Frank burned the wood which carried his life as only the flame of life could melt Thanatos' chains. Frank successfully melted the chains thereby freeing the god. With Thanatos free, any enemy that was killed would stay dead as Thanatos told them he would prevent anyone that was dead from returning from the Underworld.

Personality
Thanatos was believed to be merciless and undiscriminating, sharing a mutual hatred for both the other gods as well as mortals. He may resent his job as when Percy, Frank, and Hazel arrive to free him, he explains that doing so could cost them their lives and he would be forced to bring them to the Underworld, prompting the other to ask if he even wanted to be free. He also explains to them that death isn't fair, but it is his duty to stop souls from returning. He may have a sick sense of humor as once he is free, he checks his list of souls and when he doesn't see Hazel's, he tells her he could call Pluto and get this oversight taken care of. However, this could simply be him doing his job. Lastly, Thanatos doesn't seem to care about returning favors. When Frank frees him, Frank asks if he would help and Thanatos said he will prevent those that die in this fight from returning, something he is suppose to do anyway as he is the god of death.

Appearance Thanatos is lean and muscular, with a regal face, honey gold eyes, and black hair flowing down his shoulders. His skin the color of teakwood, and 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, and called him 'timeless' and 'remote'. His weapon of choice -when he carries one- is the long-sickle.

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.
 * Thanatos has a list of escaped souls on his iPad. He also has video conferencing and Pluto's Skype address.
 * Thanatos is properly translated to Mors on many Greek to Latin Translators.
 * Hazel claims that when it comes to Death, the Romans let him stay greek.