the dark ones

Lilly wakes up in hell.
I mean literally hell.
Hell is like an old dark city.
She has a mark around her throat because she hang herself when she was alive.
But, she doesn't remember why...
She doesn't remember anything.
But she starts getting flashbacks and remembering things.

What happens when people in the old city starts seeing " The Dark Ones ".
The Dark Ones, are monster, creatures that feed of flesh and drink blood, they look like big black shadows and are supposed to stay outside The Walls so the citizens of hell are supposed to be safe.
The Walls, are big walls made by dark magic, that is suppose to keep The Dark Ones outside and if a Dark One gets seen too close to The Walls by anyone in the big watchtowers it immediately gets killed.

so, why would The Dark Ones master, whoever he is, sacrifice so many Dark Ones?
and why does it all start at the same time as Lilly arrives in hell?