Acheron: Flames of Tartarus

Acheron saw him kill her; the only person that had ever shown him kindness and compassion when everyone else turned their heads away in disgust.

A reckless act of vengeance that went too far had him stripped from his position as Titan. As punishment he was cast into a human body, sentenced to live on earth for eternity, cursed to live a life of solitude.

Centuries later the goddess Athena takes him in with the promise of a purpose, and thus the Athenai came to be: A fellowship of immortal custodians who protect mankind from the evil they are too ill-equipped to battle.

It's the modern world now and Acheron is still in servitude to Athena.
Fortuitously he becomes reunited with the reincarnation of his wife but old enemies begin to resurface and with the fate of the world resting in his unstable palm he becomes worried that he would lose her again.