Thanatos

Thanatos (Ancient Greek: Θάνατος) is the god of peaceful 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 Heracles for the life of Alcestis, the wife of King Admetus. He lost the match, bringing the queen back to life.

Before The Series
When Frank Zhang was still a baby, Juno appeared before Emily Zhang and her mother to warn them about how Frank's life was tied to a burning piece of wood. She also told them that Frank was fated to one day free Thanatos from his icy chains. About fifteen years later, Emily left the piece of wood to her mother while she went to fight in the war. She died while protecting her troops and Thanatos lead her down to the Underworld.

Shortly before Jason Grace's disappearance from Camp Jupiter, Thanatos was lured to Alaska, where he was captured. Alcyoneus used chains made of ice to hold down the greater part of his essence, preventing him from using most of his abilities. Gaea then used his absence to take control of the Doors of Death, forcing them open and allowing souls that would help her defeat the Olympians back into the mortal world and giving monsters the ability to reform almost instantly.

The Sword of Hades
Nico mentions Thanatos when he tells Thalia and Percy about Sisyphus, who chained him.

The Son Of Neptune
It is revealed that Thanatos had gone missing when Gwendolyn was killed by a spear through her chest, but didn't die and simply walked out of an exit door in the Underworld. Mars arrived at Camp Jupiter and issued a quest, telling the camp that Death had been captured, resulting in monsters reforming instantly and the enemies of the gods returning to walk the earth. He ordered that his son, Frank Zhang, would lead the quest, but he had to take Percy Jackson with him to teach him respect for Mars. Frank later picked Hazel Levesque to go on the quest to free Thanatos.

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.

Before leaving, Frank asked Thanatos what the Doors of Death were and how to close them. Thanatos admits that he does not have the power to close them as they are currently under Gaea's control. He does however give them a clue as to how to find them, as only a special demigod can find the doors, but they need to start their search in Rome. When Hazel asked about her brother, Thanatos said she would find out what had happened to him in Rome. He then departed to Camp Jupiter, as he felt there would be many souls there to reap.

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 supposed 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 is the color of teakwood, and his wings glimmer in shades of blue, black, and purple. He is frequently mistaken for the god of love, Cupid. Hazel Levesque describes him as the most beautiful man she has ever seen, and called him timeless and remote.

Abilities
Since Thanatos is the god of death, he presumably has control over death, control of the dead, and the ability to kill someone with a touch. As part of his duties, Thanatos is able to separate the dead from the living. He captures souls trying to escape from the Underworld and sends them back. Also, because he usually is in control of the Doors of Death, he is able to quickly pass between the world of the living and the dead.

Magical Items
He owns a pure black iPad that he uses to keep track of the escaped souls. This item is also able to use video conferencing and has Pluto's Skype address. He also owns a long sickle or scythe with a Stygian Iron blade.

Trivia

 * The name Thanatos means 'Death' in Ancient Greek, just as Mors means 'Death' in Latin.
 * The word thanatos is a term for the killing instinct.
 * 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.
 * Thanatology, the scientific study of death, is named after him.
 * Though he is descirbed as having skin the color of teakwood (a tan brown shade), Thanatos's picture in The Demigod Files depicts him with a much darker skin tone along with a much larger build.