Northstar Rock is a small area in Red County, San Andreas, located north of Los Santos. On the in-game map, however, it says North Rock not Northstar Rock. Swapping the words in the name will yield "Rockstar North," an Easter Egg and a reference to the producer of some of the Grand Theft Auto games.
A little mysterious shack is located up the hill, and just like the shacks to the northwest at The Panopticon, its floors are covered in blood.
It is probably based off of the real-life Mount Wilson, a mountain north of Los Angeles.