A secret chamber underneath Sabek Palace that houses another interplanar object. Its mirrored walls display infinite reflections.

You descend a spiral staircase beneath the seal in the great hall and find yourself in a long chamber with fully mirrored walls, floor, and ceiling. At the far end of this chamber, a durgolium chest rests on a pedestal. - Eye of Ulos (RPA-ADV4)

The durgolium chest here is guarded by mysterious knights;

Suddenly, you find your way blocked by knights in sparkling armor who seem to appear from nowhere. Push, pull, strike, or struggle, you find you cannot make contact with these knights, but they force you back to the stairs.

Once your feet are off the mirrored floor, they march back to some unseen alcove in the depths of the hall. As the knights return to their posts, you notice that they make no reflection in the mirrors that surround you. - Eye of Ulos (RPA-ADV4)

Upon solving the puzzle of the room and opening the chest, you find instead the imprint of a jar and a brooch.

You open the durgolium box, but the earth does not shake. No energy crackles over your head. There is a circular shape in the dust at the bottom of the box, where a jar once sat. Near this circle sits a lovely, but not otherworldly, brooch. - Eye of Ulos (RPA-ADV4)