Start exploring the [Judicator's Tomb]. Head towards the adjoining chamber, find a summoning circle and use Interdiction. When the Dead Lord appears, have him interact with the unreachable switch (the above screen). Proceed forth.
At the next chamber, move right and get ready for the first battle. While fighting Undead Prowlers, don't forget to dodge. A Skeletal Champion will soon join in, and due to the small size of the chamber it's more than recommended to use Death Grip and area-effect attacks.
Afterwards, examine the nearby area (weapon rack) and stand in the summoning circle to use Interdiction again. Have the Dead Lord interact with the close-by switch.
Go back to the main chamber area and jump down. Push the pillar there, placing it between the two platforms pictured on the above screen. When you do it, go back up (interactive edges).
[TIP: Ignore the two chests displayed on the mini-map, because you'll be able to reach them only after the official ending of this quest, which is the enslaving of another Dead Lord.]
Finally, have your undead helper go to the inaccessible southern part of the chamber and trigger a pressure plate (the above screen).
Now you can start thinking about getting to the other side. Begin by jumping down on the first platform (it draws out after using the switch), then climb the pillar (the above screen) and jump over to the other platform (lifted thanks to the Dead Lord standing on the pressure plate).
[TIP: If the first platform dropped down in the meantime, go back to the switch and use it again.]
Once you're on the new balcony, look around for a chest and a Soul Arbiter's Scroll. Head south afterwards. Approach the Judicator (the above screen) and listen to his conversation to learn that the Dead Lords demands you bring him three souls.