The Chestnut King (100 Cupboards Book 3) by N. D. Wilson

The bestselling and highly acclaimed 100 Cupboards series concludes with one final, epic battle in The Chestnut King. Perfect for readers who love Percy Jackson, the Unwanteds, and Beyonders!
 
Hidden cupboards behind Henry’s bedroom wall unlocked portals to other worlds that Henry and his cousin Henrietta couldn’t resist exploring. But they made one terrible mistake—they released the undying witch Nimiane. Her goal? To drain all life from every world connected to the cupboards. Henry must seek out the Chestnut King to defeat her, but doing so will force Henry to make a terrible, irreversible choice. With the fate of the worlds and everyone Henry loves hanging in the balance, will he have the courage to do what is needed to destroy the witch once and for all?
 
"A must-read series." —The Washington Post