Darkness Comes (aka Darkfall)

They found four corpses in four days. Each more hideously disfigured than the last, the bodies punctured with dozens of tiny wounds.At first they thought it was a savage psychopath. Then they thought it was a vicious gangland war. Then they thought packs of demonic rats were escaping through the ventilation system.

Then they saw the nightmare itself, in all its mottled, slimy horror, coming after them from every direction, and they realized that the Gates of Hell had been left open...