The other side.

'There's a war looming, as there has been since the start of fucking time, but for the first time, someone like Scarlett exists. So the Progeny are offspring of angels, Scarlett is a descendant of one particular angel- Death. For centuries, the offspring of Death have lived in hiding, but Scarlett's father, her real father didn't want that life for her. But of course her power is immense and no one can stifle it. That's why people around her keep dying, as she grows her power does too and soon it will be impossible to control.'
Scarlett is surrounded by death. Things happen and she doesn't understand.
Sebastian is on the other side. The wrong side. The evil side. Archetype of a supernatural bad boy.
As their stories intertwine, things get progressively worse and soon, there will be nothing left.