|“||There is no fairness in Death. If you free me, I will do my duty.||”|
Once Thanatos was tricked by Sisyphus, which was one of the reasons why the mortal king was condemned to the Fields of Punishment. When Thanatos came to take Sisyphus to the Underworld, the cunning man chained up the god and shoved him under his bed, effectively stopping death until Ares freed him.
Thanatos also wrestled with Hercules for the life of Queen Alcestis, the wife of King Admetus. He lost the match to the mighty demigod and was forced to bring the queen back to life.
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 led 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.
It is revealed that Thanatos had gone missing when Gwendolyn was killed by a pilum 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.
With the help of Bob and Damasen, Percy and Annabeth break the chains on the Doors of Death in Tartarus, something Bob tells them will restore the Doors to Thanatos' control with the severing of the chains on the mortal side. After Hazel frees the mortal side of the doors, they reset and will appear at random which will allow only Thanatos to sense where they are and keep them out of Gaea's control.
Thanatos was believed to be merciless and undiscriminating, sharing a mutual hatred for most of the other gods as well as mortals. However, Thanatos is fiercely loyal to Hades, his master, and has a good relationship with Ares, since the god of war does not enjoy bloodshed without Death, and personally freed him after Sisyphus chained Thanatos up. The god 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 dark 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.
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 dark 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.
Despite not being as powerful as an Olympian god, Thanatos is still a powerful and important god, he is able to keep track of all souls in the underworld, this way he keeps the balance between the living and the dead, and stop the world from falling in chaos.
- Necromancy: Since Thanatos is the god of peaceful death and the own death, he has absolute control over death, and the dead, as well as 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. It's thought that he was the same powers of Hades and Nico, only in a infinity extension.
- Doors of Death Control: As the god of peaceful death and the own death, Thanatos is usually in complete control of the Doors of Death, and is able to quickly pass between the world of the living and the dead. Interestingly, in The Son of Neptune, Thanatos referred to them as "the Doors of Me."
- Invisibility: As revealed in Percy Jackson's Greek Gods, after Ares frees Thanatos from Sisyphus, the god of peaceful death no longer openly approaches souls directly, and instead, chooses to reap them while staying invisible.
- Flight: With the help of his huge dark wings, Thanatos can quickly glide through the air, which helps him capture souls at very high speeds.
Thanatos 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. Thanatos also owns a long scythe with a Stygian Iron blade.
- The name Thanatos means 'Death' in Ancient Greek, just as Mors means 'Death' in Latin.
- As a son of Nyx, Thanatos is older than the Olympians.
- 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.
- Even though Hazel has stated that the Romans let Thanatos stay Greek, Letus or Mors is actually the Roman form of Thanatos.
- Thanatology, the scientific study of death, is named after him. This is the science that human Howard Claymore studies.
- Thanatophobia, the fear of death, is also named after him.
- His depiction as carrying a scythe and being 'the Reaper of Souls' is similar to many folklores' description of the Grim Reaper, a hooded skeleton carrying a scythe to 'mow down' the dead. This implies Thanatos may have inspired these legends.
- Hazel also compared him to the Angel of Death due to his big black wings.
- Though he is described as having skin the color of teak wood (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.
- Thanatos' Egyptian equivalent is Anubis.
- Thanatos' Norse equivalent is Hel.
- He and his twin brother are referenced in song in the classic novel, "The Grapes of Wrath." The line references sleep and death as brothers.
- It's thought that gods are capable of dividing his essence to being in different places at same time, Thanatos is very skilled at this as in Percy Jackson's Greek Gods he is decribed to be who go to reap the souls and independent wheres you "kill" someone, Thanatos act.
- It's possible too that he only is capable of sending his power to wherever he wants.
- Even if he is described to be the God of Peaceful Death, he's the truly god of death, being the own death, as when he is stuck people stop to dying, even if the people died in a violent way.