The Heir and the Spare by Harper L. Woods

Here is a summary with spoilers for The Heir and the Spare by Harper L. Woods

This book should be read after book 4. It is a novella. 

Mab, the evil ruler of the Shadow Court, is trying to learn more about Estrella’s powers. To break her, she captures Fallon, a fae girl who has been hiding for twelve lifetimes and one of Estrella’s closest friends, and begins torturing her. Mab holds these sessions in front of her court to reinforce her control. Fallon copes by trying to think of anything else and waits to return to her room to be healed.

During these sessions, Etan, the Summer Court prince, is present. As he watches Fallon, he decides he wants to take her to his court and make her his wife. He plans to approach Mab carefully to secure Fallon.

Estrella and Fallon are brought in together for another session, where Estrella is forced to use her magic. It is revealed that she is a primordial. Mab also tells Fallon she will be sent to the Summer Court and promises it will not be pleasant.

Etan becomes fully obsessed with Fallon and wants to protect her. He follows her and fights off other court members to keep her safe. He decides to act and claim Fallon.

Everyone is brought to the throne room, where Mab goes on a rampage. She murders Rheaghan and enslaves Estrella. Fallon cannot understand how Estrella can endure the loss of her mate. That night, Fallon goes to sleep, and Etan kidnaps her and begins their escape.

When Fallon wakes, she realizes she is no longer in the Shadow Court and that Etan has taken her. She is more concerned that her healer is not with her. Etan tells her they are already betrothed and are traveling to his kingdom to be married. When they arrive at the Summer Court, she is introduced to his people. They love her, and she grows to love them.

Etan takes Fallon to the spring so they can officially consummate their marriage before the Primordials. The gods acknowledge them, and they become royals. Afterward, they choose to share a private moment. Fallon then panics because she did not expect to develop feelings, while Etan realizes that Fallon is his mate.