Go through the door leading to the northern (starting) section of the Judicar's Tomb. In the chamber where you had to push the pillar, jump down and use the pictured interactive edges to get to the balcony on the other side.
[TIP: If you don't wish to complete the exploration of the Judicar's Tomb but instead want to focus on the main storyline, you can leave this location using fast travel. I, however, recommend penetrating the whole tomb, especially since it won't take much time.]
Turn left, stand in the summoning circle and use Interdiction to summon BOTH Dead Lords at the same time. When you do it, head east to reach the stairs shown above.
After going down to the basement, send the first Dead Lord to stand on the pressure plate on the right (screen 1), and the other one to stand on the pressure plate in front of you (screen 2). You can now access two nearby chests. Once you take what's inside, proceed to the main quest [Basileus].