The Priestess never actually appeared in a Legacy of Kain game. She was intended to feature in Soul Reaver, but because of the drastic changes that were made to the storyline prior to release, her character was removed. Evidence of her existence remains in the unused audio files on the game disks, and also in the promotional comic produced by Top Cow.
She was the High Priestess of the Vampire Worshippers. Kain was her God, and she worshipped him and his vampire offspring. She believed Raziel was a heretic for betraying Kain and his brethren. At some point during Soul Reaver, Raziel would have fought and killed her. By devouring her soul, he would have gained the ability to possess his enemies. The Elder God described the skill in this piece of unused dialogue:
"In her long service to her vampire masters, the Priestess had learned a measure of sorcery. In devouring her soul, this power is conferred on you. You may project your spirit, and in so doing, take possession of your victim's body."
As recently as January 2006, it was possible to purchase the comic online from Comic Central. It is short, but nicely presented, and gives an interesting perspective on Raziel's character prior to his execution.