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human
[Tolkien] If the Balrogs are warped Maiar, and Sauron is a Maiar, why did Sauron keep his form?

From what I understand, he kept his fair-looking form even after turning his allegiance to the Dark Lord. Why is this? Also, was he turned to Melkor's cause before or after the Balrogs? Was he always on Melkor's side?
gpt
Not all Maiar are equal, just as not all Valar are equal.  The most powerful Maia is probably Eonwe, but Sauron is pretty high up on the list, even though he is just a servant of Aule and not Manwe.

Also one of his earliest and inherent abilities was shape shifting, which makes sense as he is a servant of Aule the Smith, no doubt that just as he can manipulate and change the materials of the earth, he can manipulate and change the appearance of himself.

Edit:

As for 'sides'.  The Balrogs may just be native maia of Melkor.  All Valar have maia who are affiliated with them.  Being as Melkor is being of immense shadowy darkness made of ice and fire, it would make sense for his maia to be similar in nature.  

Sauron was not originally on Melkor's side, he was a servant of Aule, but the power and majesty of Melkor is tempting and Sauron fell to this temptation.  But he was not always evil and even repented for a time when Melkor was overthrown.  But ultimately he was too far gone for any lasting change.