Traitor of the Tides by Frost Kay

A banished sirenidae princess sold to her sworn enemy.
Mer has one goal: avenge her fallen love. But getting close to the cold Methian king isn’t so easy. She’ll have to infiltrate his court, marry him, and break his heart. But charming her gruff husband is harder than she anticipates, and Mer finds herself either wanting to kill him or fall for him.

A ruthless king who will do anything to save his empire.
A deadly plague grows in Methi, King Raziel’s only hope to save his people is a marriage alliance. He wasn’t prepared for his new bride to try to kill him on sight, nor the blinding attraction he feels for her. His court calls for her death, but Raz can’t afford to endanger the treaty. Instead, he must do something far more dangerous: make his wife fall in love with him.

A battle of wits and hearts.
When their marriage becomes a game of wills, deception, and secret societies, Mer and Raziel are pitted against each other in a millennia-old war as they play for the throne, each other’s secrets, and love.

May the coldest heart win.