Berserker Temptation by Asa Maria Bradley

He’s an undercover FBI agent, hunting for answers to his brother’s suicide.

The childhood genetic experiments that gave Luke Holden exceptional speed and strength cursed his twin with such horrific hallucinations that he overdosed to escape them. For years, Luke’s one goal has been to expose the covert research lab he blames for his brother’s death. He’ll sacrifice everything to complete the mission–even the blonde, green-eyed goddess who’s haunted his dreams ever since she left his bed after one night of scorching pleasure.
She’s a disgraced immortal Valkyrie with a last chance of redemption.
During her last assignment, Astrid Idrisdotter almost killed an innocent bystander and lost the respect of her fellow Norse warriors. Her current duties offer a chance to salvage her tarnished reputation. All she has to do is escort an important guest to the royal wedding. Easy-peasy for a shield maiden armed with modern-day weapons.
Together they are sizzling perfection. But is he playing her for the secrets she’ll die to protect?
But then Loki’s monsters kidnap the VIP and to find him, Astrid must forge an alliance with a former lover whose steel-gray gaze pierces a little too deep into her soul. Their past night together fractured the walls she built around her heart centuries ago. What if trusting him now shatters them all together?