Witch Kingdom was the home of all Witches.
Rhiannon Crochan was the last Crochan Queen and the former Queen of the Witch Kingdom. During the sieges of approximately 500 years befor Throne of Glass, She cursed the Witch Kingdom on the dawn of the fourth day with this curse: Blood to blood and soul to soul, together this was done, and only together can it be undone. Be the bridge, be the light. When iron melts, when flowers spring from fields of blood—let the land be witness, and return home.
She cursed the three Matrons, and through them, all Ironteeth. She cursed Yellowlegs herself – who gave Rhiannon her finishing blow. She cursed the kingdom so that the Ironteeth may win the war but they would never win the land. That for what the Ironteeth had done, they would inherit the land only to see it wilt and die in their hands.
In the events of Kingdom of Ash, the Thirteen sacrificed themselves to take down a witch-tower brought by Morath's army. The Thirteen yielded so that the witch-tower would melt, alongside Morath's Ironteeth witches. After the Yielding, Manon flew to where the Thirteen's Yielding happened and was on her knees, broken. People started bringing flowers to set down where the witch-tower stood, on a field of blood.
After the events of Kingdom of Ash, the curse was broken. Manon, the Ironteeth witch-rebels and the Crochans left to the Witch Kingdom to rebuild their kingdom as one people.