He was 10 when Sebastian won and he knew things were off by the way the Shadow Market was emptier, fae being carted off back to Faerie, Warlocks becoming more and more sick. His dad was more openly scared while also trying to get on Sebastian's good side. Sebastian knew he was a conman and didn't listen, he had more important things to worry about.
Then the warlocks turn to demons, all warlock magic stops and demons, now more prevalent than ever, eat Johnny alive.
Kit's so terrified that his powers explode earlier than usual. He ends up in faerie, scared and alone, when they shut it off. As the Unseelie King goes to kill the Seelie Queen, the Rider's of Mannan see Kit and go after him. He's only saved when the Wild Hunt, with Kieran and Mark, protect him.
They take him in, train him as war kicks off in Faerie. Kit gets closer to Kieran and Mark, he gets hardened by war. The Unseelie King kills the Seelie Queen, Kit learns of his heritage and thinks he should have a say.
Gwyn tells Kit that they don't pick sides in the courts. He tells him that there won't be any sides if he's successful. He gets kicked out but Kieran and Mark follow him.
He works his way through the Unseelie Court. Kieran uses his status where he can and they kill where they can't. Mark tells Kit of his family, Kit's jealous because his only reference is them and the wild hunt. He promises him that when he takes over, he'll open the gates to faerie and reunite him with his family.
After years of fighting, of gathering rogue fae who were never in courts, those who believe in the first heir, Seelie who want Arawn out, and those just interested in what will happen. When they get to the Unseelie King, they have an Army. The war is long and brutal bit eventually Kit wins
He's a just ruler at first, learning more and more about his powers as he harnesses those that he has naturally and ones from combining both crowns. But this is thule, and years of pain, blood and anguish, make Faerie fall under shadow just like in the myth.
They work on breaking through the gates. Kit's ready to face Sebastian, ready to take the shadow realm too, he is the lost herondale after all. When a portal opens up, he wonders what it is when he sees the Seelie Queen. Huh, he thought she was dead?