Thanatos is a demon from Shin Megami Tensei: Persona series.
Thanatos is personification 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. In the Greek Poem, Theogony, Thanatos (Greek: θάνατος — Death) is one of the sons of Nyx 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 Cupid. The depiction of Thanatos in Persona 3 seems to be derived more from the earliest mythological accounts, being characterized as a grim warrior surrounded by a mantle of metal coffins.