The Place where Raziel would have met the vampire worshippers in the human city and The high Priestess of the Vampire worshippers.
The Undercity is one of the deleted areas which were accessed originally from the Human Citadel. The City was a hunting ground for the Vampire Worshippers which abducted the citizens to serve as sacrifices. The Elder God warns Raziel of corruption spreading throughout the city as Raziel needs to obtain possession ability to get to the smokestack area.
The Undercity was originally accessed through City 9, which in retail version houses a different corridor, leading into a larger room with two blocks and a dead Dumahim. By flooding the room, Raziel is able to stack the blocks and access entirely different looking room labelled as Mrlock 7 (staircase) and Mrlock 8 (fountain room), housing a health upgrade in the end of the fountain room. However, these two areas were supposed to be in a different location altogether, possibly suggesting near other Mrlock rooms found in the released version. The pc version has leftover rooms City 10 and 11 which load separately and are revealed to be laying right next to City 9. By inserting City 10 and 11 into the corridor, it provides a perfect match. City 10 is a short corridor with a battered gate and leads to a large water reservoir chamber with a single pipe jutting out of a wall. This pipe would have presumably lead to the Undercity.
The Undercity was a large set of rooms deep underground. The puzzles encountered were regarding flooding certain corridors/main bridge room in order to get to the top to fight the Priestess. Raziel would have to systematically pull levers to open floodgates as he progressed further up. Once at the top, he would have to encounter the Priestess at her temple to gain the possession ability.
The Undercity consisted of a central room with several bridges attached to the stone towers in the middle.