What are these outsiders in game terms? A solar and powerful nature elemental lord? If they could be slain by 20th level PCs, a CR of 24+ would be my guess.
If you are refering to Rirru and Corsentius: their exact nature would be anyone's guess. Rirru is a fiend, though that says little of what it is in game terms, except that it's evil and hails from Hell. In the Horror Vaccui setting, "Fiend" is the name of something which comes from Hell (or more specifically, something which was created with Hell). Fiends come in every size and colour; every fiend is unique. There are no balors, lemures, or vrocks. Indeed, there are no demons, devils, or yugoloths. Fiends share only three traits:
* Fiends are evil, but they can be either lawful, neutral, or chaotic.
* Fiends were created with Hell (a fiend can never perish as long as Hell still exists).
* Fiends share a few game-statistics (See the MM, baatezu qualities).
The same with Corsentius, who hails from Heaven. As a celestial, he is completely unique (no planetars, solars, etc). Celestials also share three traits:
*Celestials are good, but they can be either lawful, netural, or chaotic.
*Celestials were created with Heaven, but they can be slain (and thus their souls are banished to the Void). This puts the forces of good at a
severe disadvantage compared to the forces of evil. Why there is such an inherent lack of balance in the multiverse is unfathomable.
*Celestials share a few game-statistics (see the MM).
I won't give Rirru and Corsentius complete statistics as of yet, I've got many more important things to do with the Horror Vaccui setting.