Thanatos

Thanatos was the primordial God of Death, an ancient and fearsome entity who predated the Olympians and ruled the Domain of Death as the reaper of all souls. At the bidding of Zeus he imprisoned Deimos, and when Kratos came to free his brother years later, the God of Death killed Deimos before Kratos destroyed him. (God of War: Ghost of Sparta)

The reaper of souls#

Thanatos was described as an ancient and fearsome Primordial, an entity that predated the very cycle of life and death and stood as the reaper of all souls. He ruled the Domain of Death, a place few gods or mortals dared enter and from which fewer ever returned. He was a solitary and enigmatic being who rarely left it or troubled himself with the wider world, yet he was cruel and sadistic, and he held no respect for the younger Olympian Gods. (God of War: Ghost of Sparta)

The imprisonment of Deimos#

Believing Deimos to be the Marked Warrior prophesied to bring about the destruction of Olympus, Zeus, Ares, and Athena sought to remove the threat. Thanatos agreed to take Deimos and hide him where he could not be found, and so the young Spartan was torn away and imprisoned in the Domain of Death, tortured by the god for years. Long after, the true Marked Warrior, Kratos, having slain Ares and become the new God of War, learned from his mother Callisto that his brother still lived, and he set out to find him. (God of War: Ghost of Sparta)

The death of Deimos#

As Kratos journeyed toward his brother, Thanatos possessed a corpse to warn the Spartan against pursuing his path any further, preferring to keep the status quo than invite confrontation. Kratos pressed on regardless and slew the god's daughter, Erinys. When at last he reached the Domain of Death and freed Deimos, his brother attacked him in rage for having failed to save him years before. Into the midst of their struggle came Thanatos, who snatched Deimos away, reminding Kratos that he had killed Erinys and vowing to kill Deimos in return. (God of War: Ghost of Sparta)

Thanatos carried Deimos to Suicide Bluffs, the very cliff where Kratos had once tried to take his own life, and cast him over the edge, declaring that the oracle had lied about the prophecy. Kratos saved his brother at the last moment, and the two made peace and resolved to face the God of Death together. Seeing the red tattoo and the ashes of Kratos' wife and daughter bound to his skin, Thanatos realized that the oracle had spoken true after all and that Kratos, not Deimos, was the Marked Warrior, remarking that Ares had chosen poorly. (God of War: Ghost of Sparta)

The fall of the God of Death#

Thanatos fought the two brothers with his curved blade and then took the form of a great winged, armored dragon. In the heat of the battle, Deimos saved Kratos from the god's grip only to be smashed into the rocks and killed. Enraged, Kratos shattered the monstrous armor until Thanatos, mortally wounded, reverted to his humanoid form. Kratos beat him down and was met only with taunts, then tore open the god's cracked chest and impaled his heart with his blades, ending his life. Before perishing, Thanatos taunted the Spartan one final time and gave him a devilish grin as he exploded into dust and light. That duel is the climax of the Ghost of Sparta plot. (God of War: Ghost of Sparta)

Afterward the mysterious Grave Digger told Kratos that by his victory over Thanatos he had become Death, the destroyer of worlds. The Grave Digger is widely taken to be Zeus in mortal guise, and the novelization of the first game all but confirms it. If so, the King of the Gods now stood face to face with the true Marked Warrior, the man who would one day bring Olympus to ruin. (God of War: Ghost of Sparta; God of War (novel))