I'm reading through Descent into Avernus and my vision of it is a Mad Max-esque world. Red, desolate, almost like a desert but not classified as one. The mountains there have no vegetation… just more rocks.
For Ranger's Natural Explorer, and I play Adventurer's League so Rules as Written, would any terrain work in this environment? I feel like the most accurate description would be Tundra. But the closest option is Desert.
Best Answer
Avernus is a desert
The best descriptions of Avernus I've found are in the Baldur's Gate: Descent into Avernus adventure, under the "Paradise Lost" heading in the introduction (p. 9):
And in Mordenkainen's Tome of Foes, under the "Lady of Ruin" section about Zariel's realm (p. 11):
It is only described as a wasteland, that used to be lush and beautiful. Combined with its tendency for spontaneous mirages and the direct comparison to Mad Max, it's a desert.