A compelling update of a medieval 'romance', which kept me reading not just for the suspense but also for the moving love story. If you've no idea of the novel's basis (the story of Sir Gawain and the green knight), it won't interfere at all with your enjoyment, but should increase its unpredictability.
It isn't the darkness causing it. Those memories are safe to explore. It's the moment earlier when he nearly lost it in front of Kat, when the story he was telling got too close. He wants to go there, to probe and assess the danger level. But his mind won't let him. Repels him like a negative magnet. He has no control. However much he wants the future to work, he is powerless against the past, which could creep in unexpectedly and fuck it up at any moment.