'Walkers' by Graham Masterton. VERY creepy!
http://www.amazon.com/gp/product/0812522095?ie=UTF8&tag=whatsthatbook-20&linkCode=as2
Publisher's Weekly
In Masterton's latest, quite effective horror novel, Jack Reed, who runs a
muffler shop, comes upon a ''castle'' that serves as the setting for this
gruesome horror story. Jack decides to turn the abandoned mansion into a
resort--not that he really knows how. He knows even less about the Druid
magic that allowed the recent occupants, dangerous mental patients, to
''escape'' into the building's walls. Led by a vicious brute, Quintus, the
''earth walkers'' kidnap Jack's son, Randy, and demand the return of the
priest who had trapped them in 1926. The priest is ''persuaded'' to free
them from the confinement of the castle's grounds, but they will not be
freed from the earth until each has killed 800 people as sacrifices to the
gods. Wasting no time, the tribe ''walks off'' for a murderous orgy of
killing, dragging their screaming victims into the ''underworld''--where
Randy is still a captive. Readers who fancy unrestrained terror won't mind
indulging in the wild suspension of disbelief that Masterton (Mirror)
demands
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"Jack Reed comes across his life's ambition in the woods of Wisconsin: an
old derelict building that was once a sanitarium known as The Oakes.
Abandoned for almost 60 years, Reed sees potential in it as a future resort
or country club. He learns very quickly, however, that it harbors dark and
dangerous secrets and it has chosen him to open it's Pandora's Box of
horrors. 135 patients vanished inside the place in the 1920's and they have
been living inside it's very walls. Not behind the walls, mind you, but IN
the walls. They will stop at nothing to free themselves, including
kidnapping Reed's own son"
