The slot allocated to Rancid Earth in Torment was originally going to be used for a creature enchantment, but 11th-hour playtesting showed that black needed another way to deal with the horde of squirrels green was capable of producing. And so Rancid Earth was born.
Art had already been commissioned for the set at this point, and nothing was suitable for a land destruction card. Luckily, the original art for Tremble had been forced out of Odyssey due to some other necessary swaps, and it was without a home.
The original Tremble art showed some dwarves beset by some sort of land destruction, but the effect wasn't "black" enough. So the crack art team added in an evil spell-like effect, and the finished product is the card you all know today.