Luster is the region that is home to the unicorns and is linked to Earth through seven Gates, or portals. It acts as a parallel universe, and can only be reached through the portals or one of the Queen's amulets. These amulets grant the means of entering Luster. The unicorns' meet at four distinct places located on the four corners of the continent. These places, which are inhabited during the season whereafter they are named, are called Springdale, Summerhaven, Autumngrove, and Winterkeep. Throughout, the unicorns migrate to these areas depending on the season.
Some humans have found their way to Luster including Alma Leonetti, Thomas the TInker, and Cara Diana Hunter Seven known dragons live in Luster: Firethroat, Graumag, Fah Leing, Red rage, Elliban, Bloodtounge, and Bronzeclaw. These were the last seven dragons on Earth, who fled to Luster when (according to the story's premise) they were unable to remain. Other races include dwarves, delvers, and centaurs.
Between Luster and Earth, and presumably between other worlds, lies a realm of light in all the colors of the visible spectrum. Because this realm is sometimes used as a place to where exiles are sent, it is called the Rainbow Prison. It is accessible by means of certain jewels which correspond in color to individual "shafts" of the Prison.