His Obsession

Assassin X Assassin
He's cold, lethal, and commands every room he enters-but she makes him falter in ways no one else can. Kaylynn is giddy, fearless, and alive, and he's been tasked with training her personally. Thrust together under the organization's strict rules, they must navigate missions, deadly enemies, and their overcomplicated relationship. In a world where one misstep means death, the strongest assassin finds himself dangerously distracted.

For the dark romance part of the story, I tried to have a lot of tension build up until one of them finally breaks. The more time he spends with her and works with her, his obsession and possessiveness grows. He values control, especially over her.