Thanatos

Thanatos is the god of darkness, murder, and undeath. He is the archnemesis of the Raven Queen, and has repeatedly attempted to defeat her and usurp her pantheon and restore the throne of the god she destroyed.

In ages long forgotten, the being that would become Thanatos was a powerful daemon lord, but when the Raven Queen defeated the god of the dead and replaced him, the intervention of the gods left a significant portfolio, that of undeath and the terrible darkness of oblivion, open for the taking. The daemon managed to take control of the portfolio and defeated all who would try to take it from him, and thus Thanatos, the Death of Gods, was born.

Thanatos appears as a massive, gaunt man with a chest that decays into a rotten, organless cavity below his ribcage, long curved horns, and eyeless sockets that constantly weep an oily, black smoke that curls up into the air between his horns.