The Vanished Ones
The Vanished Ones were the non-original inhabitants of Mirrodin that originally were caught by Memnarch's soul traps and disappeared again at the end of the Fifth Dawn (for example Raksha Golden Cub and the Leonin seer Ushanti).
This event was known as "The Vanishing". Almost all of the remaining trolls, half of the elves, and a third of the humans disappeared without a trace. They returned to the planes from which they had been brought to Mirrodin.