Ultimate Persona of the Death arcana in [[Persona 3]].  Summoned momentarily when the protagonist first summons [[Orpheus (persona)|]] in the early game and later can be fused and summoned regularly after completing the Death social link.  

The mystery of Thanatos's sudden appearance is central to the plot, though this is not apparent until much later.  Accordingly, Thanatos along with [[Orpheus (persona)|]] are the two most common of the protagonist's many Personas that appear in artwork involving him/her.

[spoiler]Actually, it is Death itself who emerged in the early game, having been sealed in the protagonist by Aegis many years ago when he was a child. Generally speaking, this tag should be used for either character as they are nigh-identical in appearance, differing only slightly in color scheme.[/spoiler]