Zathal is entirely unique as a material plane in the fact that the only survivable realm is the Underdark.  The surface world was made entirely inhospitable to any kind of life due to the Great Cataclysm. A number of the surface races died out due to their inability or refusal to adapt to the Underdark enviorns, while a large number of the survivors found themselves enslaved to the races of the Lands Below.

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