FRPG thought; any non native in the Underdark deals with depression and dietary problems due to lack of light. Elves straight up die there without trees or seasons, and the word for dark elf doesn’t mean “traitor,” but literally “demon” for what else could dwell in such a place? Many others- bird or cat folk - simply won’t go that deep. Humans and even the tough halflings face penalties until they break through, quickly they hope, to the deepest levels, warmed and lit by geothermal energy.