Wrath and Crystal Thorns - Fae and Crystal Thorns, #5

The king of Crystalfall has returned, plunging the court into chaos. Chloe and Quintus are desperate to defeat this malicious ruler and end his reign of terror. But the king's power and ingenuity make him the greatest enemy Faerie has ever known. Even worse, the king is planning a sacrificial ritual, which will give him even more power. If the king succeeds, his magic will be strong enough to destroy all the other courts in Faerie.
He will instantly crush any court that attempts to defy him. And he'll get rid of his son for good. Chloe and Quintus must stop the king before he gains this ultimate power. If they don't, Faerie itself will be ruined forever.