Obsidian Sanctum is a Supreme Grade Domain, granted to Alan by his darkness affinity, from his rightful position as a prince of Death. However, it is a very distinct, or, different domain than others.
The Jade Palace is a truly Unique Domain, but Obsidian Sanctum is special in it's own right. Domains are, even at the Legendary Rank, trump cards one uses only when they are pushed to Death's door. They are a trump card that must only be used when there is simply no option left.
Not only are domains tailored towards their user, but they are almost always extremely costly, even Dragons sometimes struggle with keeping their 'Domains' up for longer than usual.
Domains are the very existence of the user being pushed from their soul, to establish a foothold in the real world. In a sense, you could say that Domains are the manifestations of the User's will, forcefully establishing a foothold in the real world.
