Fate of the Autumn Queen (Autumn Queen Trilogy #3) by Savannah Stoltz:
 
Princess Aurelia Amberheart’s plan to challenge her aunt for the throne of the Westlands has proven disastrous. Now, the only way to avoid fighting her sister in a duel to the death is a marriage to the one male she can’t seem to live with or without: Finneas Ravencrest, her first love and the man who betrayed her.

But even with the support of her loved-ones and the Winter Kingdom, the outcome of the trials is devastating, leaving Aurelia more desperate than ever to remove her tyrannical aunt from power.
With the gods and goddesses active again after thousands of years and war looming on the horizon, Aurelia must enlist help from her friends and family to defeat Queen Selena and ensure a life free from oppression for the people of Iaselion.

Brimming with red-hot tension, heart-pounding action, and a realm-changing battle, Fate of the Autumn Queen is the third and final installment in the unforgettable Autumn Queen Trilogy.