Tower of Thorns by Jenna Wolfhart

Cursed are those who kill a king.
Reyna and Lorcan have won the war. But for how long?
The death of Unseelie’s king has unleashed a curse across the realm. Soon, every fae alive will end up bound to its dark magic.
Meanwhile, the allied courts refuse peace, convinced the curse will spread north.
Haunted by heartbreaking bargains and terrifying prophecies, Reyna must finally face the truth about the godlike magic inside of her. She might be the only one with the power to shatter the curse and unite the courts once and for all.
And if she fails, darkness could rip apart the very fabric of her world.