They're fighting against Voldemort Valentine,
a powerful Shadowhunter with a circle of like - minded followers who were eager to purify the bloodline until a brief war seemed to kill the villain — right after he and his followers had the children who must now save the world.
Morgenstern's character is another, very
powerful Shadowhunter, but one who's gone a little bit too far in his quest to purify the world.