A sithen is a faerie mound, the home of the fae. Once whole worlds were contained in the underground realm, but faded with the loss of magics among the fae.
The coming of the Sidhe to America during the Jefferson Administration caused them to create new sithens in the Cahokia Mounds in Illinois, directly across the Mississippi River from modern St. Louis, Missouri.
The sithen have powerful magics to keep them inaccessible to non-fae, but sometimes the sithen will welcome human visitors out of malice or playfulness. They are protected by fairy plants, objects of terrifying beauty, and living guardians. Each major court; Seelie, Unseelie, Sluagh, and Goblin, maintains their own sithen.
After Meredith created an extension of faerie on Maeve Reed's estate in A Lick of Frost, it became possible for her and Rhys to create a new sithen. When they did so, it appeared as an rundown apartment building in a bad neighborhood, but it was Rhys' sithen.