Maiar
The Maiar were spirits of the Ainur, lesser in might than the Valar but of the same order, who entered into Arda, the world, to aid the Valar in its ordering and governance. Like the Valar, they had existed before the making of the world, born of the thought of Eru Ilúvatar, and they took part in the great Music from which Arda was conceived. Each of the Maiar was associated with one or more of the greater Powers, serving and learning from them.
The Maiar could take on forms visible to the Children of Ilúvatar, fair or terrible as they chose, but their true being was that of spirit. They were many, and most go unnamed in the histories, but several played great parts in the fate of Middle-earth -- for good and for ill.
Notable Maiar
Among the Maiar who served faithfully were:
- Eönwë, herald of Manwë, who led the host of the West against Morgoth at the end of the First Age
- Ilmarë, handmaid of Varda
- Olórin, wisest of the Maiar, who came to Middle-earth in the Third Age as Gandalf the Grey, an Istar
- Melian, who wedded the Elven king Thingol and set her girdle of protection about the realm of Doriath
- Arien, who guided the vessel of the Sun, and Tilion, who steered the Moon
Others fell into darkness. Greatest and most dreadful of these was Sauron, originally a Maia of Aulë the Smith, who was seduced by Morgoth and became his chief lieutenant, and after Morgoth's fall the second Dark Lord. The Balrogs too -- demons of shadow and flame, such as the one that destroyed Gandalf in Moria -- were Maiar corrupted by Morgoth in the ancient days.
The Istari
In the Third Age, five Maiar were chosen by the Valar and sent into Middle-earth in the guise of old men to oppose Sauron. These were the Istari, the wizards -- among them Gandalf (Olórin), Saruman (Curumo), and Radagast. Clad in mortal flesh and forbidden to reveal their full power, they worked by counsel and encouragement rather than by force.
Significance
The Maiar weave through the whole history of Middle-earth in both their fairest and foulest forms. From Gandalf, who gave his life and labored selflessly for the free peoples, to Sauron, whose lust for order and dominion became the great evil of the age, they show that the same high origin may lead to salvation or to ruin, according to the choices of the will.