Prisoner of Darkness by Nissa Leder

How far would Scarlett go to save someone she loves?

Scarlett returns to the mortal realm different than before. She’s mended her relationship with her sister, but now has new secrets she must keep. After taking the pill Kaelem, the Unseelie King, had given her at the Summer Court, the magic in Scarlett grows, and she’s losing control of her power.

When Kaelem offers Scarlett his help, she has no choice but to accept before she hurts someone she loves. Scarlett must trust him to teach her to master her new gifts, but his intentions aren’t pure and Scarlett is pulled even further into the fae world.