How I Stopped Worrying and Learned to Love Eberron
In the order of things, powerful creatures usually have some sort of lesser race that assists them. Sharks have lamprey eels, the sharks benefiting from the cleaning power of the eel, while the eel obtains food from its work. The same applies in the realm of magic. The original demon lords had the rakshasas as servants, and were so connected to them that people eventually started calling them rakshasa rajahs. The dragons have the dragonborn. And though few of them still exist, the couatl have the deva (and to a lesser extent, the shulassakar).
The deva were once a prolific race during the Age of Demons. As the forces of good scrambled to keep back the demon lords from destroying the world, they developed tools and armies to assist them.
One of the most interesting questions about the deva goes back to the question: which came first, the chicken, or the egg? Some deva claim that a life of evil will cause a deva to be reincarnated as a rakshasa. Others claim that they are former rakshasas, redeemed through good works, and reincarnated as deva. Are the two races connected? How?