It is believed after defeating Kain in the the Chronoplast and gaining form him the Shift-at-Will ability, that Raziel would have turned his attention to the Priestess and, passing through the City, would have entered the Undercity and Temple, the lair of the Priestess. Daniel Cabuco confirmed that the entrance to the lair was to be entered through the second water pipe leading downwards in the main Water Tower courtyard area of the City. There he would have fought many Vampire worshippers until facing the Priestess herself who would have summoned 'monsters' to fight Raziel. Upon defeating her he would have gained the Possession ability and freed the Citadel from her influence.
It was previously believed that the depiction of the woman to be the Priestess in the Temple area of Soul Reaver, however, this appear to be unlikely due to the area being decorated with Sarafan imager and artist Daniel Cabuco confirmed that the depiction was just an ordinary woman.
Another believed depiction is the Legacy of Kain: Soul Reaver title screen which features a number of stylised statues said to represent the lieutenants. The Priestess was initially said to be among them, but Daniel Cabuco late clarified that the staue represented Malek.
Though she does not appear in Soul Reaver, there are images of her in concept art and the Legacy of Kain: Soul Reaver comic.
The legacy of the Priestess has continued throughout the series: Soul Reaver 2 originally featured a Vampire worshipping cult, that was removed (this led to the Feral humans appearing in Legacy of Kain: Defiance). Defiance also originally had Umah fulfilling a similar role to that of the Priestess (before she became a vampire). This was also removed.
Given the likelihood that Kain must eventually return to the era of his empire, the possibility remains that the Priestess will return in her original form.