Beings from the outer planes are those who originate from the planes of the gods. These beings are rare in the world of Altosole as the outer planes have long since been closed off. With the retreat of the gods however many beings of the outer planes were left stranded on the mortal plane. They are technically considered immortal beings who return to their home plane when destroyed instead of being lost or transitioning into stray souls. Since the outer planes have been closed off, when a being of the outer planes is killed they will return to their home plane still, but can never again return to the mortal plane until the return of the gods. The only hope to return a being of the outer planes after death is to intercept them in the Mists Between and prevent their passing into the outer planes.
Note that those beings left behind on the mortal plane are limited in power either because of their low rank among the outer planes or because they were cut off from their source of power. There were historically much more powerful beings in the outer planes capable of even bending reality. These beings are rumored to sometimes influence the world (as NPCs) and form religions of their own.