Okay, try this one in Camera Raw. Select the Adjustment Brush tool (K) which is the 10th icon from the right.
In the slider box click on the minus sign under brightness a couple of times, then drag the cursor along the joining line in your image. This should darken the light area. Get the circle of the cursor quite large. You will see a + inside a solid line circle and a dotted line circle. Only what the + touches will get darker, if the plus sign hits areas you do not want darkened, the plus sign will darken the area. In other words it is okay if the circles cross the dividing line, but try to avoid having the plus sign cross over. The circles give you plenty of warning and allow you to move and bounce the cursor along quickly.
Once you have it darkened once, now go back and play with the brightness and exposure sliders to darken or lighten your work.
You could also try just the opposite and lighten up the dark line of the hills.
To show what you have worked over just check the "Show Mask" square and you can select a color if you wish. If you click on a pin with a black dot, the results will show, too.
This tool is a good way to lighten up or darken an area of an image quickly using Camera Raw.