I'm really liking what I've seen of Bestiary 5 so far. The art in particular continues to floor me. With that said, here are some mostly unrelated questions.
1) Would you say aether elementals are less common than the other "main" elementals in their respective planes, or about the same? As far as I can recall they haven't been mentioned as common denizens of the ethereal plane before, whereas by all accounts fire elementals are everywhere in the plane of fire.
2) Are there procedures that exist for a mortal turning themselves into a devil, similar to the demon transformation rituals from book of the damned 2? Or is the devilbound creature template and other contract-granted stuff the local equivalent of that? What about for the celestial races, can someone become a half-celestial with enough devotion? And I imagine daemons wouldn't be very interested in granting that sort of thing.
3) What does negative energy damage look like (in the context of inflict spells, that new void kineticist, etc.)? Does it leave visible wounds or is it more of just a "draining the life force" thing?
4) Why is the Vilderavn so cool?