Lilith - The Mortal Instruments Series
Lilith is called by several names, but the most common is: "The Mother of Demons". So you can blame her for the Demons, not me or anyone else.
Lilith was originally meant to be Adam's wife, instead of Eve. (Adam and Lilith doesn't have the same kind of ring to it, if I'm honest.) But Lilith wasn't. Lilith wasn't obedient to Adam nor God (If you follow that kinda stuff...) so she was cast out of Eden and replaced with Eve.
Lilith was punished for this severely, she is unable to bear children, or, at least, she was only capable of gestating infants who died upon their birth. Lilith is associated with the harming and weakening of babies.
Soon after, she went into a relationship with the Fallen Angel Sammael, and their relationship corrupted her into the first Demon to have originally been human.
Later on, Sammael and Lilith traversed the Earth, sirring up Demons as they went, promoting the creation of Warlocks as they went.
Weapons and Powers:
Lilith has many abilities which she can use.
By using her blood and the blood of someone else, you can bind two people together. The pain of one, the other shall feel. It's dangerous and fatal. You kill one, the other will die.
Her blood can be used to change people, for example the Nephilim. You give them the blood of Lilith in the Infernal Cup, they'll turn dark.
How to Kill:
There are no problems with killing her. You won't have to. She is unable to leave The Void therefore there is no need to kill her.
She is classed as a Division Five due to her strength, power, history and what she can do to people, given the correct circumstances.
Be strong my friends, do not waver your sword of justice.