You can unlock this quest during a conversation with the Chancellor, who resides in the [Eternal Throne] in the Kingdom of the dead. Selecting Help each other? in the dialogue (the above screen) will get you a task to explore the Soul Arbiter's Maze.
20 Soul Arbiter's Scrolls have been scattered all around the game world, and their purpose is only one - to help you navigate through the Soul Arbiter's Maze. Written on them are directions that suggest which way to go, which you can check out in your Inventory (press TAB when the souls are highlighted - the above screen). Note! If you wish to follow the walkthrough detailed below, you can treat collecting the scrolls as an optional challenge - the game guide will tell you exactly which way to take in order to make your way through the subsequent levels.