Tread Softly a.k.a. Dark Mountain is a novel by Richard Laymon first published in 1987. It was republished with Beware! in Volume 4 of the Richard Laymon Collection.
Summary[]
Two families join forces for a camping holiday in the California mountains. It's meant to be fun - a break from city life, a healthy interlude in the hills amid the wonders of Nature. If only they hadn't pitched camp at Mesquite Lake, home to two of the wilderness's most terrifying inhabitants - an aged hag whose loyalty to the Evil One gives her gruesome powers, and her son, a depraved half-beast whose unnatural lusts even she connot control...