The Undeath were creatures that roamed Eden in the millions before the concept of death existed, therefore being immortal and undying. However, The Undeath were also beings that did not possess souls, and were little more than walking husks. When the Four Lords came to Eden, they brought with them the concept of death, and instilled this concept into the Undeath, giving them the ability to be killed. Within a year, The Four Lords successfully eradicated The Undeath from Eden to begin the Age of The Four Lords to shape Eden into a world of balance.
The Undeath are vaguely humanoid creatures, ranging in shape and size. Some are about human height, while others are towering colossi that cause tremors with each step. Numbering in the millions, they wandered aimlessly around Eden for millennia until being exterminated by the Four Lords.
It is rumored that some Undeath yet survive, roaming around Eden's darkest and most dangerous lands. The ones that survived are said to crave the souls of the living, and killing one is incredibly difficult and dangerous, but not impossible. If they are still around as rumors say, that would make them the oldest beings in existence.