The Dead Lands of Athas
The Rhulisti were the progenitors of nearly all sentient life on Athas. They are the direct descendants of halflings, who have all but devolved into the savages. They were masters over nature, using their advanced technology (known as Life-Shaping) to alter living organisms into all manner of useful roles, or create new life from scratch. The Rhulisti form itself was sacred and all were forbidden from altering their perfect form.
At some point in their history, whether by accident or malicious intent, something called the the Brown Tide was formed — a contagion that spread through the oceans, killing all life in its wake. They managed to stop it, but the world they knew was forever changed. In the aftermath of this event, they were forced to evolve to survive -bringing their race to an end and spawning the new races.