Came across an article which posits a theory far more eloquently than I could on how externally perceived beings (spirits, angels, aliens) can naturally emerge in the brain.
http://www.meltingasphalt.com/neurons-gone-wild/Theory extended to psychedelics: Psychedelics naturally split up previous alliances/cabals among groups of neurons (possibly through decoherence: the self starts losing its grasp of the lower modules as ego death is approached). New alliances form, leading to the birth of new agents.
One could also see how in times of extreme stress (aka low amount of resources/neuromodulators), alliances can also break down, leading to the emergence of new agents (God told me to do it).