There were two ways to build a Domain.
One was provided by the Spell, and depended on the Citadels.
By becoming the master of a Citadel, or having a person who had accepted your authority become one, a Supreme could empower and expand their Domain over vast territories. A Citadel was much more than just a stronghold containing a Gateway — it was a daunting construct of impossible sorcery that forcefully subjugated the part of the world surrounding it and brought it under the rule of a Supreme.
That was why both the King of Swords and the Queen of Worms, who controlled most of the human territories in the Dream Realm, were so immensely powerful.
The second way was innate to each Transcendent, and unique to their Aspect.
Nephis suspected that most Domains were territorial in nature, which was the reason why the Spell had based its method on territory.
But not all of them were.
