The Raven Queen's predecessor

This deity was the God of Death preceding the Raven Queen. He was the father an Empyrean with a human mother. During the Age of Arcanum, the god of death punished his child by banishing it to the planes of Pandemonium with the intention of eventually free the Empyrean from that plane.

Before he could retrieve his son, a mortal mage, whose name was lost to history, crafted the rites of ascension and challenged the God of Death. She killed and destroyed him and took his place in the pantheon, becoming the Raven Queen. As of 812 PD, the previous God of Death was largely forgotten about.

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.