This kingdom is located across the Sea of Vastness . In order to get there from the Hoolian kingdoms, one must travel from Beyond the Beyond and across the Sea of Vastness.
Before the Books
Theo, the first blacksmith, followed Ivar to the Middle Kingdoms. He lured the remaining hagsfiends in the Hoolian kingdoms to the Middle Kingdom, where he trapped their minds with vanity and illusions of power. Theo would be known as the first H'ryth.
The Middle Kingdoms are a main component in this book. It is where the chaw of chaws traveled. They found here a race of owls that resembled those from other kingdoms, just that they were blue (i.e., the Striga) and called dragon owls.
In Exile, the Striga is the main antagonist. In an effort to take control of Coryn's mind and the Tree, he bans the Band from the Tree. He came from the Middle Kingdoms.
Tengshu (from the Middle Kingdoms) in the end comes to help them.
In The War of the Ember, the Dragon Owls came from the Middle Kingdoms to fight in the war (with the Pure Ones and the Striga.)
Also, Coryn wished to hide the Ember in the Middle Kingdoms. (This never ended up happening as they cannot get permission from Theosang.)