Dark Mountain — Richard Laymon

For two families, it was supposed to be a relaxing camping trip in the California mountains. They thought it would be fun to get away from everything for a while. But they're not alone. The woods are also home to two terrifying residents who don't take kindly to strangers--an old hag with unholy powers, and her hulking son, a half-wild brute with uncontrollable, violent urges. The campers still need to get away--but now their lives depend on it!