The Price of Magic by Alaina Stanford

Nicole wasn't from this world. She didn't know a quest for the Sword of Eternal Fire was a death sentence. If she had known, she wouldn't have cared.

Nicole earned the title of Elf Warrior through her fierce deeds. Her husband Jack, always fought by her side. He was her calming influence, the love of her life, and the father of her twins.

Regardless of the twins’ half-human blood, they would easily live a thousand years... just as their mother would. But their father was mortal. He'd be lucky to reach eighty. Nicole's heart ached for him. Then, it began to burn. Burn like the Sword of Eternal Fire... The only talisman in existence that could extend its owner's life.

There was only one problem... well, two actually: the sword is hidden in a lost Elf city, and no one ever returned from a quest for the sword.