Master of the Shadows (Darkyn #7.5)

Will Scarlet, right-hand man to Darkyn leader Robin of Locksley, has been charged with an impossible task: He must regain control of his lord-s stronghold, which has been invaded by vicious Darkyn renegades, without harming any captives or attracting the attention of the Brethren, the Kyn-s mortal enemies. Things only get worse when Will learns that Reese Carmichael, the mortal woman he loves, is one of the hostages.
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Reese was sent by her father, a former Brethren inquisitor, to infiltrate Rosethorn, retrieve an ancient vial containing the cause of a medieval plague, and prevent Armageddon from being accidentally unleashed upon the mortal world. Seducing and falling in love with Will Scarlet was never part of the plan, but Reese has no choice other than to use him and the siege of Rosethorn to complete her mission. If she does, she knows he will never forgive her-but if she doesn-t, millions of innocent lives will be lost.
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As Kyn allies Jayr, Byrne, and Lucan arrive to provide Will with reinforcements, Reese-s true identity and motives are revealed.- Torn between love and loyalty, Will and Reese uncover one last secret from the past that may help them win the final battle-if it doesn-t destroy them first.