The Rasa were once Asag's servants, primordial lifeforms born of the fires of the world before Yevna claimed the world for her own.
These nightmarish evil spirits persisted even after the Lords Alliance cast Asag's forces out of Arcadia. Those who remain in Midworld have been encased in the mortal bodies of monstrous beastmen. Little is known of their society (or even if they have one), other than that they tend to rule secretly and from the shadows, especially in the Dune Sea of Mamara, corrupting the minds of the Sultans there with promises of power and riches.
The Rasa are immortal like the Yevim, but there is only one generation of them that is constantly reborn and re-implanted into new bodies. It is rumored that the Rasa possess the same shapeshifting abilities as the Changelings, except that their ability to change shape allows for perfect accuracy.