I think all the muscles turned out really well - at least I'm very happy with them! I also think the eyes turned out pretty good. For the "dragon horns", I tried to add some highlights with a lighter red, which worked pretty well as long as you don't look at the figure too closely.
For the base, I used colored sand again to create a kind of volcano and lava. In any case, this is how I imagine a demon/hell dimension. The skulls were of course absolutely necessary to make Samael look really evil ;)