Awakening Magic

So I need a magical phenomenon that is:

- Prone to being awake/asleep
- can be awakened by blood/murder
- can fit seamlessly into an urban fantasy
- is not a ley line

I might have an idea that's currently set on the back burner, and the plot basically relies on magic that somebody/a group of somebodies really want/would kill for, and one such death leads to it waking up.

Well, to take a page out of Teen Wolf’s book, you could have bloodthirsty trees with a power that’s only awakened by the murder of a witch/wizard/any character you want on like a specific day or a made-up supernatural holiday.

To add more to that idea, the forest has been asleep for hundreds of years, ever since the last death that only partly waked it up because it was suicide or someone sacrificing themselves and the trees can tell the difference. And the power build-up is the largest it’s ever been because it was dormant that long.

Did you want like a real example? Oh well. Hope that makes sense XD.