Two things happen when two formidable power collides, either destruction ensues or the creation of something new occurs. 

Moon Goddess must be high on something while pairing them together. Or maybe she knew only they can complete each other. She is the protector of her people, he raids through villages, slaughtering people left, right, and center to fulfill his greed.

She is the Royal queen. He is the chieftan of barbarians. Guess who rejected who. 
He left her on her knees, heartbroken. She begged, she cried, she howled for him but she also rose. Despite a broken heart, she rose to become the queen she was.       
7 years later. The queen of Avalon is admired widely for her beauty, strength, and intelligence. The only thing she lacks is a heart. She makes the ruling look easy with her practiced elegant smile. But it is not, especially when there is an army of vampires rampaging through the nations, killing werewolves for power. In the mid of the crisis, her advisors suggest they form an alliance with the most feared clan, Barbarians. The barbarian chieftain agrees to help them but on one condition. 
He wants an heir from his mate in return.

https://booknet.com/en/book/barbarians-rejected-luna-b396867