My take on it:
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When the Astral Plane, the realm of thought and concept, condensed, individual concepts, ideas, abstractions, ideologies, paradigms, and memes filled the silver void. Some of these were all-powerful, mighty and sufficiently self-aware to grow, expand, and form an active agenda - these are the deities living in the Astral Plane.
However, the deities are not encompassing all of these entities, many are too abstract, too minor, or too narrow to coalesce into a self-aware materialized being. Myriads of these motes are floating through the Astral Sea.
However, these motes are attracted to similar concepts, broader ideas, and encompassing thought constructs - just like individual ideas fit into a thought paradigm. Due to this, the motes wandered towards to like-minded gods, who were in tune with their own nature.
During the Primordial Wars, the gods quickly saw the need for soldiers and helpers. In their need, they took motes similar to their own being and endowed them with a tiny bit of their own, more powerful essence, enough to coalesce into existence - and created the angels.
The angels are usually aligned with their gods, but since they are only ideas close to their divine ruler's nature, they are not mere extensions of their divine will, but independent beings, acting out of their own volition, serving the gods, because they are so close to their gods' ideology, hence fulfilling their own desire to embody their ideal.
The power and rank of angels is normally determined by the match of the godly spark and the basic nature of the mote - the better they are in sync, the more powerful the bond between the metaphysical entities is - hence the less energy is wasted in to bond and the more powerful the angel becomes - which is also the reason, why higher ranking angels are often knowing their god's will much better than angels of lower ranks.
However, since they are now self-aware beings, they can - just as gods - subtly shift their own nature - which is basically the gift of being a sapient entity. When gods undergo paradigm shifts, some of the angels are often no longer compatible to their god's portfolio - in such cases, two things can happen:
The angel dissolves itself, splitting into a spark of the godly soul (which is re-absorbed by the god) and the original mote (which often gravitates towards another, more kindred god) - or the angel wrests the divine spark free, which is the concept of falling.
In such cases, the mote and the divine spark are fused together brutally, despite the fact that they're incompatible - hence such fallen angels are often marked by a violated appearance, reflecting their contradicting nature.
Normally, gods (and also angels themselves) only undergo very slow paradigm shifts, hence such angels do not fall, but decrease and rank and often dissolve willingly, before the fall happens (as they are still close enough to the god to respect him and his will). In fact, gods actively try to keep the paradigm shift in reign, after the spectacular Fall of Asmodeus:
He was an Angel of the highest rank, but when his god underwent a sudden shift, he went below the lowest rank - no longer close enough to his superior, he wrested the essence free, which such a force, that the deity suffered a large metaphysical wound. Since he was not the only angel suffering such a fall, he became the leader of the rebels, trying to kill the deity.
They, of course, knew what would happen - they would either fall even more or their divine essence would vanish with their dying god, dissolving them into mere motes. Unwilling to suffer that fate, Asmodeus reached into his deity's soul and tried to overtake it, to fuse with it. And he succeeded, becoming a self-contradicting and hence wholly corrupted being.
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Cheers, LT.
