The Spirit World is a mysterious alternative world in The Crown & The Flame series. The Fire-Users at the Blackspine Mountains are closely related to this place. It is first seen in Book 2.
Not much is known about the Spirit World, but Anu Rhuka mentioned that whoever travels inside this world, will meet spirits, some dead, some living, and some whose status is unclear.
History
Entering the Spirit World is no easy task, it requires a great amount of training and control over emotions in order to enter, but most of all, a spiritual center is necessary.
Initially, it was believed that only Fire-Users were able to access this strange world, but it was later revealed that even ordinary humans can enter it, through extensive training.
Characters who have ventured in the Spirit World later recalled that it was a strange, but beautiful experience.
One should always be careful while in the Spirit World, not all spirits are harmless, and some will attempt to harm whoever is journeying inside the world.
The spiritual center varies from person to person, but most of the Fire-Users share the same center, a volcano.
Kenna Rys, however, had a different center, her home, Stormholt, most likely because her home meant everything to her.
In order to enter the Spirit World, Fire-Users sit down on fires, close their eyes and breathe calmly, humans share a similar method, that only requires closing the eyes, focusing on breathing, and listening closely to the sounds of the environment, like the wind blowing.
Fire-Users usually venture in the Spirit World in order to unlock their true potential, their "dragon".
The Crown & The Flame
Dominic Hunter visits the Spirit World for training of his fire powers with the help of Sei Rhuka and Anu. On one of his visits, Dom is there with his Hawk that carries a fiery orb and if you whistle at it it releases the orb in his hand, which ends up in the Real World and when it opens a phoenix comes out. In the later battle, he awaken his Dragon Form alongside with Sei. In The Crown & The Flame, Book 3, Chapter 17, Kenna was trained by Dominic in a premium choice to enter the Spirit World to meet Leon Sterling, Gabriel Amarne, and Adrianna Rhys to tell them what had happened and for a chance for her to say good-bye and thank you for what the heroes had done for her.