Now I am not an expert on the AD&D system of powers and only know so much of Aber-Toril. However, I am an expert on Tolkien, from who all FRPGs draw much inspiration, perhaps the entire fantasy genre, that can be debated.
Think of the powerful spirits in Tolkien's universe (I shall refrain from using the specific names of various places;-) ), including Elves, BTW. Their form is a manifestation of their will so to speak, and/or Eru's will. All of them reform when killed, think of Sauron's death at the downfall of Númenór and his subsequent reforming, this was admittedly aided by the One, but according to the mythology behind it (I think it is ine one of the History of Middle Earth books), powerful spirits could reform when killed. Just as Melkor will at some stage be able to return from the Void he was banished to...
Further, consider that all Tolkien's evil "Deities" (Powerful spirits) such as Melkor/Morgoth, Sauron, Gothmog and the other Balrogs are actually the same kin as the "Good" spirits, but chose wrong and followed an evil path (seduced by Melkor and in his case by ambition), and thus became evil. They are still of the same kin as those faithful to Eru (and one can claim that they too are faithful to Eru's balance as everything, even their evil is borne of his thought). Now, the Valar are not strictly speaking deities, but would be understood as such by the men of Middle Earth and any other medieval world, and we can thus label them such. This would give a translatable system of understanding (and stop me rambling), if we percieve it such.
Middle Earth Catholic Earth Aber-Thoril
Eru "The One" God
(you tell me)
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The Valar, Manwë, Archangles I suppose Gods
Elbereth, etc Including the fallen Including evil ones such
MELKOR/MORGOTH SATAN as Bane, Cyric, Myrkul and Bhaal
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Maiar; Olorin/Gandalf Angels and DEVILS Demigods god and evil
Saruman, Eonwë, etc. I suppose DEVIL and DEMON princes
Including SAURON and the
BALROGS
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Lesser Spirits such as
Oromes Horse Nahar,
Caradhas, WIGHTS