Helheim is the realm of Hel, the realm for people who did not die in battle. This was our interpretation of it in our game Apsulov: End of Gods.
Most things are sculpted in zbrush, to later be textured in engine using masks with material made with a mix between Quixel mixer with megascans and Substance Designer. Some assets like the cliffs are megascans meshes with a different tiling material ontop.