The Hounds are supernatural creatures of an unknown nature, seemingly summoned by blood rituals. In keeping with that theme, they seem to hunt by the scent of blood.

Overview

I knew that they'd done something pretty big to use that much blood, and I tried to tell them about the herpes, but they wouldn't listen. I was walking home after school, and I heard a noise like barking behind me, but —
  — A partially burned letter written by Sadie Brown as part of her therapy

Highly involved in the Sadie Brown case, the Hounds were supposedly summoned by a boy who had gathered around him a cult whose spells were based on sex and blood. When Sadie tried to warn the other girls in the group about a venereal disease the boy may have passed on to them, the boy summoned the Hounds and set them after her.

Though Brown escaped and found sanctuary in Bishopsgate Mental Asylum, the carved saints of which protected her for a time, they continued hunting, eventually succeeding in breaking in and killing Brown while she was weakened following an extended vegetative state.

References

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