On the following image you can compare how the photo manipulation looks before and after this adjustment.

Step 17 - Desaturation

Add new adjustment layer Hue/Saturation on the top of all layers and set the value Saturation on -50.

This adjustment tone down most colors. But don't worry. In next step you'll add beautiful blue tone to it.

Step 18 - Blue Tones

Let's focus on toning image. To create more mystical, calm atmosphere you'll add blue and cyan tones in this step.  Add new empty layer on top of all layers and name it BLUE TONES. Grab the Paint Bucket Tool (G). Pick some very dark, almost black color. (I used #06000F). Fill the layer with it. To blend it properly change its Blending Mode from Normal to Exclusion.

Below you can compare how the photo manipulation looks before and after this adjustment.

Step 19 - Darkening

Add new adjustment layer Curves on the top of all layers and set it as shown below.

This shape of curve makes dark tones of your image darker. You can compare the result on the following photo.

Step 20 - Adding Birds

In this step you'll add birds to the horizon to bring more details to your photo manipulation.  Download the bird brushes listed in the beginning of the tutorial and install them into Photoshop. Add new empty layer on the top of all layers and name it BIRDS.  Select some brush you've just installed. Press Alt on your keyboard to activate the Eyedropper Tool (I) and pick some dark color from the picture. Simply paint some birds on horizon.

Step 21 - Sharpening

This is the final step when you'll make your photo manipulation sharper.  Press Ctrl + A on you keyboard to select the whole image. Go to Edit > Copy merged (or press Ctrl + Shift + C on your keyboard) to copy it merged. Go to Edit > Paste (Ctrl + V) to paste it. Place this new layer on the top of all layers and name it SHARPENING.  Go to Filter > Other >High Pass and set the value Radius on 1px. Press OK.

To blend it properly change its Blending Mode from Normal to Overlay.

Before you finish this tutorial let's take a quick look on all the layers you've created: