The Raven Queen's predecessor

This deity was the God of Death preceding the Raven Queen.

This god did not regard death as a natural transition and took a tyrannical stance over death and the afterlife. During the Age of Arcanum, this god had gained a mortal mage (whose name was lost to history) as his follower and partner, who would take his place one day as the god of death.

During the same period, the god of death punished his child, the Forgotten Empyrean, by banishing it to the plane of Pandemonium. Before he could retrieve his son from Pandemonium, his partner crafted the rites of ascension and challenged him. She felled or, by rising, instantly obliterated this deity and took his place in the pantheon, becoming the Raven Queen, though she was not permitted to claim his whole portfolio, instead only gaining power over the moment of death. As of 812 PD, the previous God of Death was largely forgotten.

Trivia

 * In the Dawn War pantheon of the Nentir Vale setting, which the Critical Role pantheon is based on, the god of death that the Raven Queen replaced was Nerull.