"We seem to have a bit in common, Casper, " Xaanik smiled, seeming more at peace around the virtue then he would almost any other stranger. "I have a question... Actually, I'd like your advice, if I may. I'd like to know how you manage to keep yourself seperate from the suffering of the humans around you, while still being able to care for them. Does all the injustice in the world make you feel helpless? How do you deal with things you can't change?"
[sorry if it's not a simple question, though I think this is something Xaanik would really like to know, in character]
Ah! That's a good question.
It's not possible for an actual angel to die or be destroyed, so defective ones are isolated to the underworld instead. However, if someone goes to the effort of summoning them to Earth and keeping them there for many years, their angelic nature wanes and it does eventually become possible for them to die/be destroyed (albeit only through a special ritual).
As angels aren't creatures meant for this plane of existence, it takes a long time for their... resonances to attune to an earthly one. But once that happens, they're no longer really angelic.... If that makes sense?
Deities don't exile defective angels directly to earth because they're often too dangerous to risk having around humans. They figure it's better to just tuck them away in the underworld where they'll have little to no contact with Earth.
It's extremely rare for humans to be able to summon fallen angels/ashfiend, and whoever was being summoned would have to play nice if they wanted to stay on earth anyway (because they'd still need that mage's help for multiple years), so deities aren't too concerned about stopping this from happening.
Seeing as Xaanik's been on earth long enough, his powers have decayed substantially and he's technically mortal now (though it'd still take a lot of effort to kill him!)
--That's actually the distinction between a "grey" ashfiend and a regular, non-grey one -- "greys" are ashfiend who have been attuned to earthly existence.
phew that was a bit of a ramble sorry omg
hope it answers the question well enough tho!