The trapdoor | scifi alien illustration by @kimal73

To create this landscape I started from sketches made with Mypaint. Mypaint is an open source software that you can safely download from the internet. In fact it is not very used and honestly I do not understand just why. It is done very well.

This bottom is a first schematic idea of work:
The composition is diagonal to create more movement.
On the back there is the fog. It creates depth. The two aliens will emerge from the fog.
At the bottom there are buttresses that are opposite to the diagonal line of the composition. The buttresses are the ruins of an architecture unknown to us.

In the figure below I zoomed in on alien figures. They are coming down towards the center of the composition where there is a strange trapdoor. This is a space-time passage that will bring these creatures to Earth.

To realize the landscape I used a software for fractal graphics processing. It acts on mathematical rules. Changing these parameters will change the 3d graph. Color is a material that fits into the three-dimensional model.
The figure below is another shot of the trapdoor.