Thanatos is the personification and god of death and mortality in Greek mythology. He is commonly known as a harbinger of peaceful death, bringing the eternal sleep of death to the world while more violent gods were responsible for harsher death. He did not play a major part in Greek mythology and rarely appeared in any stories, as he was mostly displaced by Hades, the god of the underworld.
In the Greek poem, Theogony, Thanatos (Greek: θάνατος, Death) is one of the sons of Nyx and Erebus and is the elder twin brother of Hypnos. In the earliest mythological accounts, Thanatos is the harbinger of peaceful death, bringing eternal sleep to the world. Characterized as indiscriminate and merciless, Thanatos is detested by all and he hates them in return, all too happy to bring mortals to their fated end. Thanatos is perceived by Greek poets as a fearsome, sword-wielding, shaggy bearded specter and fierce of countenance. He is a harbinger of suffering and grief, and his coming was marked by pain.
In later eras, Thanatos is portrayed again as an angel of death, associated more with a gentle passing than a woeful demise, guiding mortals after their death. Many Roman sarcophagi depict him as a winged boy, very much akin to Eros. His Roman counterpart is Mors, who was known through his later gentler depiction.
Thanatos can be found in the central area of the Ingress in the Womb of Grief, but due to the layout of the level, he can only be reached after the protagonist as explored a vast majority of the Sixth Sphere, while requiring the assistance of several apps along the way, including Floor Search. Like other demons sealed in the Womb of Grief, freeing Thanatos will unlock him for fusion.
In battle, Thanatos is aided by eight tentacles. These tentacles are the only enemies in the game not classified under an Order. Once Thanatos has been defeated, it will assume the form of an eyeball which can only cast the Hell Eyes skill.
Thanatos is the sole member of the Reaper Order in the PlayStation version of the game. His Order was changed to Tyrant in both the Japanese and English versions of the PSP port.
"The Greek god of death, he is the son of Nyx and the brother of Hypnos. He is depicted as a young man with an inverted torch and wreath or butterfly in his hands."
—Persona 3 compendium
Thanatos is the ultimate Persona of the Death Arcana in Persona 3. It first appears near the beginning of the game, when the protagonist first attempts to summon Orpheus. Thanatos emerges from Orpheus' head, ripping Orpheus apart and mercilessly slicing apart the Arcana Magician looming ahead. Later, the protagonist can gain accessibility to Thanatos after the Social Link with Pharos is completed.
Thanatos' fusion prerequisite requires all the Personas of the Death Arcana, namely Loa, Pale Rider, Samael, Mot and Alice to perform a pentagon-spread fusion in the Velvet Room. Thanatos also grants the equipment Ring of Darkness, which repels Darkness-based attacks, as his Heart Item.
Ryoji Mochizuki's true form as Death looks identical to Thanatos, seeing that Thanatos can be the manifestation of Ryoji's friendship with the protagonist during his form as Pharos.
Thanatos' role and attribute remains relatively the same from the original game. However, the fusion prerequisite now requires an additional Persona, namely Ghoul, an exclusive Persona of the Death Arcana in Persona 3 FES to perform a hexagon-spread fusion in the Velvet Room.
In the playable epilogue of Persona 3 FES, titled the Answer, Thanatos can also be summoned in the Velvet Room by Aigis through normal fusion.
Thanatos is one of the DLC Sub-Personas, which can be downloaded for $2. His unique skill is Door of Hades, which hits all enemies with a heavy Almighty attack, with a high chance to instantly kill, once per turn for three turns.
Thanatos appears alongside Pallas Athena, Izanagi and Messiah as one of the Personas featured in Lord of Vermilion Re: 2, once again serving as Elizabeth's Persona. Thanatos enables Elizabeth to use Megidolaon, which is considered a Physical Ability in the game.