I'd say Elder Ones, since they are all different parts of the planet and sky. I was thinking instead of outsiders, they could just be epic great wyrms with the elder one template. Maybe have Siberys as a platinum dragon, Eberron as a titanium dragon, and Khyber as a polychromatic dragon.
What would the Progenitor Wyrms from Eberron be?
doctorbadwolf
Heretic of The Seventh Circle
So the secret answer to this in my Eberron campaign is that they are the cosmos.
Siberys is the planar orrery and order/rationality/physics, Eberron is the material plane, and Khyber is chaos and magic.
Beefermatic
Hero
I'd say they're probably at Eschatolic Level
doctorbadwolf
Heretic of The Seventh Circle
In scale, yeah for sure. In my campaign the PCs recently were given a vision by Eberron, of the three creating cosmos after cosmos, in infinite variety, and having to destroy each in turn as the Daelkyr consumed them. Finally, Khyber figured out what was needed, and communed with Siberys on the matter. The two agreed, and Khyber tore Siberys asunder, making the orrery itself, and then allowed Eberron to envelope and “imprison” her to create the world of mortals, with Khyber, and thus chaos and choice and arcane power, to dwell in the heart of the mortal world.
What was missing the previous times was the Three themselves. They needed to give themselves to creation so that mortals could have the power to stand against the Daelkyr, and so for the first time the cosmos beheld dragons, and arcane and divine magic, and the draconic prophecy.
