I did have a really good plot idea, but I hsall not tell you, because then you will steal it.

.....

...ahh, screw it, here it is.

So you've got your world, and these demons that attack it. The world is basically used to it, but suddenly they start being a hell of a lot more vicious. Cue female lead, part human, part demon. Demon leader tried to control her, but then male lead saves her. Then go on a quest to detsoy damon leader, with female lead occasionaly going nuts with power and destroying stuff.

.. basically.