Most of the carvings of the altar are faded. What little remains depicts a scene of Uthenera. While interacting with the altar, such as kneeling and praying, placing a filled earthen jug, or leaving offerings (coin), the Warden may feel some sensations if his/her actions are approved or not.
This object is used in order to enter a locked chamber in the northeastern part of the Lower ruins. See this walkthrough in order to unlock the chamber which is required in order to complete The Mage's Treasure quest. If you make a mistake in the sequence, while interacting with the altar, two shades will appear and attack the party.