Tread Softly

Tread Softly is also known as Dark Mountain.

This book was later published as by Richard Laymon.

Two families join forces for a camping holiday high 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 cannot control...