Maiar

[obloquy]

Only the Valar and Maiar are intelligences that can assume forms of Arda at will.

That's from HoMe X: Morgoth's Ring - Myths Transformed. The key point is "at will."

However, the more a "clothed" Maia spirit did that was a privilege of the incarnate (i.e. eating, and especially reproducing, etc.), the more they would become bound to their physical form. It seems there were certain other circumstances that revoked this ability as well, such as the evil or good of the spirit. This is from the Silmarillion:

...[Sauron] became the greatest and most trusted of the servants of the Enemy, and the most perilous, for he could assume many forms, and for long if he willed he could still appear noble and beautiful, so as to deceive all but the most wary.

and this

There now he [Sauron] brooded in the dark, until he had wrought for himself a new shape; and it was terrible, for his fair semblance had departed for ever when he was cast into the abyss at the drowning of Númenor.

Morgoth also lost his ability to change his appearance, although I can't remember why it happened. Can anyone else find it?

[ December 25, 2001: Message edited by: obloquy ]