Shadow and Ice by Gena Showalter

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Knox of Iviland, the most ruthless warrior in All War history, has spent his life fighting others to the death, winning new realms for his king. Now the stakes are higher than ever. If he prevails in the battle for Earth, his slave bands will be removed. Victory is his sole focus…until he meets her.
On an arctic getaway with her foster sister, street tough but vulnerable Vale London is shocked to stumble upon a cave filled with ancient gods waging epic combat. When she is inadvertently drawn into their war, she is forced to team up with an unlikely ally—the sexy god who makes her burn with desire.
Though Vale is his enemy, Knox is consumed with lust and a fierce need to protect her. But only one combatant can triumph, and he will have to choose: live for freedom, or die for love.