Nightshade by Stephen Leather

In Jack Nightingale's world - where reality and the occult collide - sometimes the only way to fight evil is with evil.

A farmer walks into a school and shoots eight children dead before turning the gun on himself. It's a harrowing but straightforward case - until police search the man's farm and unearth evidence of dark Satanic practices. When the perpetrator's brother approaches Nightingale, adamant that his brother was set up, it's clear that something even more sinister lurks at the heart of the case.

And there are dark forces elsewhere. A young girl miraculously returns to life, claiming she's spoken to those from beyond the grave. Those in contact with her are dying hideous deaths . . . forcing Jack Nightingale to make the hardest decision he's ever faced