The Isle of Mist/Mist Court
The Isle of Mist is a very isolated and secrective Seelie Court. As Amber said "The Mist Court fae are, like, the most hermity hermits you’ll ever meet." They are known to be traditional fae and are cruel to the Aos Sí. It is located about halfway up the faerie continent off the east coast on an island and borders the Unseelie lands.
From the books
The Isle of Mist was technically part of the Seelie Kingdom, but we rarely had contact with the secretive Mist fae. Even so, they were known to be cruel to the Aos Sí and sent their tithes regularly. My father had never felt the need to flex his power there.***
Near the bottom of the map, the cartographer had drawn a crown. Probably Rahivea. Far north, about halfway up the continent on the east, sat an island surrounded by swirling lines. The note below it read: The Isle of Mist.