King of Frost by Ana Calin

When Lysander the King of Frost and I united our powers against Xerxes the Fire King, something unexpected happened—we became bonded mates. A catastrophe, considering that Lysander is promised to another High Fae in exchange for military support in the upcoming war. The stakes—if Lysander loses his allies and therefore the war, Xerxes will turn the human realm into hell on Earth, before he takes over all the other realms.
This means that our bond is forbidden, cursed, a tragedy. I’m trying to kill it, but seeing Lysander every day, royal and deadly, moving around like a sleek feral, it’s driving me crazy with forbidden desire. I should leave his Court, take distance, but I can’t, because Xerxes is after me.
So when an alternative presents itself, I grab on to it with both hands. It’s dark, and bloody, but it might just save the world. This means that I have to tap into the dark powers of a more obscure side of my bloodline. Problem is, these powers might kill me.