The Unique Pool is the location where Unique Items go when activated automatically by the server for entry into the game. There is a 50% chance that the Unique Item (when activated) will go to the Unique Pool; there is an equal chance that it will instead go straight to the Black Market to be sold to the highest bidder. Items in the Unique Pool simply wait to be claimed via the methods explained here. When a person rolls the appropriate number and claims a Unique Item (via a method such as a Treasure Chest), it is chosen randomly from the available items in the Unique Pool. If there are no items currently in the Unique Pool, then the chance to claim a Unique is nullified and no item is granted. The only exception to this is on the Beta and Lightning Servers, where the mage will claim any Unique Item at random, regardless of who possessed it.
When a Unique has been consumed, it is possible for it to re-appear in the Unique Pool.
I.e. When a Dead Sea Scroll has been consumed. It is possible for the Dead Sea Scroll to be found through a Treasure Chest.