Thanatos is the strongest Grim Reaper and one of the oldest.
Thanatos wears a skull mask and an exquisitely decorated cloak, the standard of all Grim Reapers.
Thanatos is calm and reserved, even giving his opponents respect when he complimented the transportation magic circle technique used against Loki, used on him, as being quite a clever technique. When his comrade Pluto was defeated by Vali, he commented that Pluto shouldn't have felt dissatisfied with touching the power of a Heavenly Dragon, showing his small fascination with fighting powerful opponents. He also has a rebellious side because as mentioned by Orcus, he hardly follows Hades' orders and often takes actions against Hades’s will. Despite his rebellious side, it was all just merely to avoid Hades finding out the secrets of creating artificial Super Devils and hates Hades of doing something nasty things again. This can be assumed that his intentions are good, yet his actions are still harsh and out of the line.
Thanatos was born from the union between the Greek primordial sibling gods of darkness, Erebus, and Nyx, sometime prior to the series.
Red Dragon Emperor of the Blazing Truth × White Dragon Emperor of the Morning Star: The True Dragon(s) of the Kuoh Academy
In Volume 24, Thanatos sent his subordinates to capture Koneko to obtain her hair clip which contains research data on creating Super Devils, to try and prevent the data from falling into Hades' hands. He officially appeared himself later on above Fafnir Stadium while the match between Team Rias Gremory and Team White Dragon Emperor of the Morning Star was going on, due to his subordinates attempts to interrupt this match having been thwarted. After he appeared above the stadium, he was transported to a wilderness area a far distance away, trapped inside a barrier erected by Rossweisse and Grayfia, engaging in a fight with Issei who was waiting for him. During the fight, Thanatos was outmatching Issei until Issei was aided by his grandfather Juzo, Guatama Buddha, and Guanyin, where Thanatos was finally defeated by Issei's Nyuutron Beam Cannon.
Powers & Abilities
Immense Strength: Thanatos is regarded as the strongest Grim Reaper among every single Ultimate-Class Grim Reaper. A simple cut of his scythe could cause great terrain damage within the space barrier. Ddraig noted that Issei's Cardinal Crimson Promotion was absolutely no match for Thanatos, and even if he were to have used Partial Dragon Deification, things wouldn't have changed much. In fact, Thanatos' power is stated to rival even that of some God-class being and was able to keep up with Issei in his DxD form.
Master Scythe Wielder: Being the strongest Grim Reaper, Thanatos' skills in wielding scythe is exceptional among the other Grim Reapers.
Immense Speed: Thanatos is extremely fast, being able to create several after images of himself and surpass Issei's speed in his Pseudo-Diabolos Dragon God form, being able to seal Issei's cannons within a matter of seconds
Replication: Thanatos has the ability to make many duplicates of himself to attack his opponents.
Magic Expert: Thanatos has some skills in magic, as he used a sealing technique to seal Issei's wing cannons.
Flight: During the events of volume 24, Thanatos has demonstrated the ability to stay afloat in mid-air.
Black Scythe: As any Grim Reaper, Thanatos' scythe can wound his opponents and shorten their lifespan with a special type of curse but as the strongest of them, his is much stronger than that of the other Grim Reapers, even Pluto's. His scythe itself is also clad in a potent aura.
- In Greek mythology, Thanatos is the personification of "Death".