The Underlord stage is achieved in three stages from Truegold. The sacred artist reaches the peak of their power and is able to open their soul space, an alternate dimension that is nested within the sacred artists channels that can store a single item. The sacred artist can then begin weaving soulfire, which is various types of aura condensed into a gray flame and stored in the soulspace. The final stage of advancement to Underlord is when the sacred artist understands why they practice the sacred arts at the subconscious level. The sacred artist can then call on their soulfire and the surrounding aura to refine the body, burning away defects and strengthening what remains. The process of advancement can take anywhere from mere moments to weeks depending on the amount of ambient aura.
"[If my memories of Northstrider have taught me anything,] Dross said, [it’s that the sacred arts only get weirder.]" Underlord