This is located in the north of the map. You will get to the west entrance of it through entering the gate of the female bandit camp. To get there just go north until you reach a big river. Follow it to the west until it turns south. Then go north again and follow the path up the mountain. Go around the tower and climb up the gate. There is also a lever to open the gate. From there go west again and then follow the map to the north. In the northeast direction of this areal is the west entrance of the Soulflayer Canyon.
If you have any questions, then feel free to ask me :)