Nemisis was overrun by demons bent on destroying the Raanaar and gaining power. What did you expect it to look like after a prolonged war?
Hey, it may have been a wasteland even before the demons invaded. Maybe the Raanaar think green is too bright and garish. That could be why they never visit Rivellon, even though there are human traders and soldiers going back and forth.
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There is a fair bit of lava and dark rock in act 2, but there are also a couple mushroom forests (though still in earth tones) and a bit of green in a couple small areas. Act 3 also has dark rock, and a burnt forest, but less lava. There is a crystal forest for a bit of colour (though the ground is still dark).