The Raven Queen's predecessor

This deity was the God of Death preceding the Raven Queen. During the Age of Arcanum, 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 Nentir Vale setting, which the Critical Role pantheon is based on, the god of death that the Raven Queen replaced was Nerull.