The Darkest Lie (Lords of the Underworld, #6)

Forced to his knees in agony whenever he speaks the truth, Gideon can recognize any lie—until he captures Scarlet, a demon-possessed immortal who claims to be his long-lost wife. He doesn't remember the beautiful female, much less wedding—or bedding—her.

But Scarlet is keeper of Nightmares, too dangerous to roam free. A future with her might mean ultimate ruin. Especially as Gideon’s enemies draw closer—and the truth threatens to destroy all he’s come to love....