The Iron Sword by Julie Kagawa

As Evenfall nears, the stakes grow ever higher for those in Faery…

Banished from the Winter Court for daring to fall in love, Prince Ash achieved the impossible and journeyed to the End of the World to earn a soul and keep his vow to always stand beside Queen Meghan of the Iron Fey.

Now he faces even more incomprehensible odds. Their son, King Keirran of the Forgotten, is missing. Something more ancient than the courts of Faery and more evil than anything Ash has faced in a millennium is rising as Evenfall approaches. And if Ash and his allies cannot stop it, the chaos that has begun to divide the world will shatter it for eternity.