The Danger in Justice by Sue Wilder

What kind of man loves… beyond vengeance?
He knows how far he will go to save her. But how far will she go… to save him?
Christan was the origin myth for the most feared creature in the ancient world, an enforcer and shapeshifter to whom all others were compared, living in a world of violence until he bonded to a mortal woman to save her life. He has loved–and lost her–for centuries. Now that bond has invited an evil into the world, a dangerous power that returns to the immortal world whenever chaos threatens. If Christan becomes Vengeance, Lexi–as Justice–will be at his side.
Lexi has realized more than the immortal world is at risk. The man she loves is willing to sacrifice his life to save hers–unless she can solve an ancient murder and defeat those who want to destroy Christan. But the dangers are greater than even she imagined and Lexi must make the most devastating choice in her life.
Because there’s a danger in justice that can destroy them all…