Nightwalker

Soldier Tatiana Douglas returns from military service in Iraq to learn that her twin brother, Lance, has died in mysterious circumstances. Her investigation reveals Lance had been a "connect"-a human who serves a demon in the hope of eventually being rewarded with eternal life. Tatiana's search leads her to Nathan Li, Lance's killer, and to X-Blade, a demon hunter. Nathan is a powerful demon and high-ranking member of the House of Cain. Smitten with Tatiana, Nathan turns her into a demon by injecting her with his own blood. Tatiana is then approached by X-Blade, who injects her with the serum he uses to control his own demonic instincts, offering her a chance to avenge her brother's death and help him bring down Nathan and the House of Cain. X-Blade reveals that Lance was helping him with a sting operation, and he and Krista form a reluctant partnership. The remainder of the season follows Tatiana's attempts to maintain her cover in the House of Cain while struggling with her growing unholy nature, and Nathan's work on the Nova Project, intended to develop a vaccine to render demons immune to their traditional weaknesses (crosses, iron, and holy water). It is revealed that Nathan has sabotaged the project and created a virus that is deadly to "truebloods"-the ruling demon class-and leaves unscathed the "turnbloods"-demons who were once human. With X-Blade's help, he eventually unleashes his weapon in the series finale.