Paint.NET :: How To Create Fade Effect

I've used Paint.net for quite some time, using it mainly for two-tone graphic art or pixel art. Quite a while ago, I created this cool effect that I'll demonstrate with images, however, I have literally completely forgotten how exactly I did it.

Anyway, here's the walk through process of the effect, it's a fade in, making it seem like there are clouds at the bottom of the silhouette that cover the black of the lower layer while doing so in a way that is like a gradient, but of course, not so linear and straight -
 
Here's the first image, ignore the shaft of light: 
 
The 2nd image shows the silhouette, the shadow of a colossus, if you will. What I want to do is add something that blends the black to the background, so you can't see his feet: 
 
The 3rd image here shows the first layer of fade (and the first layer only), this layer doesn't directly cover up the lower part of the silhouette but it does add some sort of cover for the lower parts of the main silhouette to create a more subtle blend as opposed to straight black over dark green: 
 
This is the 4th image detailing the 2nd fade-layer, still separate from the 1st fade layer just to show them separately: 
 
And finally, the fifth image showing both fade-layers, the silhouette and the background color: 
 
Ok, so that's that. Basically, how did I do that? What did I actually do? I've been playing around with the image for quite a long time, and the whole reason I'm doing this is because I'm recreating the image in a much higher resolution but I want it to be perfect, of course, and want to recreate and remember this effect.
 
I think, but am not sure, that it may have had something to do with the "Clouds" option, found under "Render". Maybe the Gradient option and blurs.