The Dominion are a set of thirteen (twelve in action, one in judgement) deity-like beings that are chosen and replaced every 12,000 years or so, on a rotating basis, meaning that every 1,000 years one of them will be replaced.
Each Dominion represents a specific function, or thing within nature that must be carried out, whether it be a Culler, an everpresent Dominion-type that represents death and change, a Jester, a less frequently incarnated Dominion-type that represents satire, humor, and mocking, or even a Wretch, one of the rarest Dominion-types that embodies great evil, corruption, and destruction from the inside out. There are many other Dominion-types as well.
The function of the Dominion is to maintain that the Amnionic Universe and Dream-circle are not harmed, and that the beings within it are able to continue dreaming to fuel the Dream-circle, to ensure that their dreams do not become wholly stale and all the same, nor that they are replaced fully by nightmares. Dominions exist to keep the order of all things in the universe the way they are meant to be, though within reason as to not drive dreams into homogenization.
Chosen Dominions have no choice in what Dominion-type they will ascend to, as it is based on a combination of the aspect of the Card they used to achieve Dominion, their alchemical balance and positioning, and their personality and beliefs.
Where do obsolete Dominions go once they are replaced? Their wisdom and spirits shift directly into the conglomerate Judge Dominion, which is not really a being, but a great, virtually endless hall in which all past Dominions debate, argue, and come to conclusions about what is to be done by the twelve in action. The Judge Dominion is the only Dominion that is never replaced, only added on to.