The Lord of the Rings: Shadow of the East
The Maiar comprise the vast majority of the Ainur who left the Timeless Halls and entered Eä. When the fifteen Valar ventured into Existence and accepted wardship over the newborn World, the Maiar followed. Maia groups ordered themselves as vassals of the greater Ainur and labored according to guidelines of their Vala masters. They set about shaping Arda as the Powers directed. The Maiar’s role was, as always, to aid their lords in completing the scheme envisioned by Eru.
Although they serve the Valar, the Maiar share the nature of their greater brethren. They too are immortal, and they too are spirits without need of form. Like the Valar, Maiar adopt corporeal bodies to interact with Eru’s Children. This enables them to perform their primary task, which is the tending of Arda. The Maiar carry their masters’ word and serve as intermediaries between the Valar and lesser beings. Since their role both permits and requires them to interact with the inhabitants of Middle-earth more often and more directly than do the Valar, Maiar take form more frequently and maintain their forms (Q. “fanar”) for longer periods. Their stronger connections to form reinforce their stronger links to lesser peoples. The Maiar, while born before Existence itself, share many of the passions and shortcomings of the Children of Ilúvatar.
The number of Maiar is unknown; however, these lesser spirits make up a true and considerable community. The Maiar are numerous enough to nurture their own societies and raise their own, magnificent war-host. Their variety is great and their tale reflects this varied character.
As Ainur, the Maiar are essentially immortal spirits, souls that are originally fully severable from any form they might adopt. Their bodies, while not required, enable them to interact in Arda, for a physical being is essential to a complete experience in a physical world. Even while at home in Valinor, the Maiar maintain form, walking among their Elven compatriots.
The Mortal Lands exert an even more physically-oriented force than Aman. Life in Middle-earth is tied to spirits which are completely interwoven with form. After all, Endor was the birthplace of Eru’s Children and remains their home. The land reflects the nature of the Free Peoples, all of whom require bodies. (Even the immortal Elves require form.) Although they are perhaps only transient residents in Middle-earth, they are Children of Arda.
The Maiar are not of Arda; instead, they entered Eä as caretakers—servants of the higher guardians, the Valar. Yet Maia spirits, while originally free, can become entombed in form. In every physical act of creation outside the scheme conceived by Eru, a part of the creator is tied to the physical world; and this rule holds true for Eru’s Children and the Ainur alike. All the Fallen Ainur gradually became tied to their bodies. As they sought to manipulate the World they were entrusted with guarding and cultivating, they became a part of that world and suffered its weaknesses. Nowhere was the danger greater than in Endor. This change occurred in Morgoth himself, as well as his Maia underlings—notably Sauron and the host of Balrogs. Tom Bombadil and Goldberry also became became rooted in form (as well as wedded to a specific area in Middle-earth.) Later, it affected the Istari.
All Ainur are immortal, of course, and the destruction of their form merely serves to sever their spirit from their corporeal bodies. Without a body, however, a Maia cannot affect the physical world, except in some cases indirecdy. In the time that it takes a Maia to reassume form, he is effectively apart from Arda and outside the concerns of Eru’s Children.
This was the case with Sauron. The Dark Lord’s form was destroyed twice in the Second Age, and each time he was unable to bring his power to bear upon Middle-earth until he could take a new form. With the destruction of the One Ring, his physical link to Arda was shattered, and he could never again assume a body. Accordingly, the Lord of the Rings passed out of the World. His soul did not die; it was simply incapable of affecting or remaining in Arda.