Knight of Darkness by Kinley MacGregor - Lords of Avalon

KNIGHT OF DARKNESS by KINLEY MacGREGOR
Lords of Avalon book

For centuries, I've been the assassin for the infamous Merlin, even though the woman who birthed me sits at the right hand of our enemy, Morgan le Fey. Now my mother has decided it's time I take her side of this conflict.

Normally, telling her no wouldn't be a problem. However my mother gave me a harsh choice: join Morgen's side or see an innocent die. I'm all for saving the innocent, but Merewyn isn't as innocent as she seems. Still, leaving her to Morgen is rough, even for me, and the only way to save us both is to face the evilest forces ever known—my mother and Morgen. Now two people who know nothing of trust must learn to rely on each other or die.