"The Keep" is a novel written by
F. Paul Wilson and published in 1981 which was later
transmogrified into a film by
Michael Mann (as
The Custodian has provided in his
writeup). This storyline of this novel is the first in a chain that Wilson refers to as
The Adversary Cycle. These novels also intertwine with his
Repairman Jack series at points ("
The Tomb" being the first of these). This is the "origin" story (at least in the modern timeframe) for
Rasalom,
the primordial evil entity that serves as the
antagonist.
To get a feel for how the novel and the movie relate, think of "
Do Androids Dream of Electric Sheep?" (by
Philip K. Dick) vs. "
Blade Runner." There are just enough similarities to keep that feeling of
deja vu without actually bothering to keep the same plot.
Tangerine Dream provide the musical score for the film version.
"The Keep" was republished in paperback format in March of 2000.