Though the Istari came secretly and in humble form, in the beginning, before the arrival in Middle-earth, they were mighty spirits. They were Maiar, spirits older than the World itself, and of that first race that came from the mind of Ilúvatar in the Timeless Halls. Yet in the diminished World of Middle-earth in the Third Age they were forbidden to come forth in power as Maiar. They were limited to the form of Men and the powers that might be found within the mortal World.
Although five Istari are said to have come to Middle-earth, only three are named in the histories that have come to Men, for the others were said to have gone to the East and they played no part in the faith of the Westlands.
First named and most praised in the Fourth Age was Gandalf the Grey, who by the Elves was called Mithrandir, by the Dwarves Tharkűn, and Incánus by the Haradrim. As a Maia in the Undying Lands he was called Olórin and was accounted the wisest of his people. At that time he resided in the gardens of Lórien, the Master of Visions and Dreams, and also went often to the house of Nienna the Weeper. Tutored by the Vala Lórien in the gardens, Olórin's wisdom for many Ages grew greater still. Also, counseled by Nienna in her house, which looks out on the Walls of the Night, to his wisdom was added pity and endurance beyond hope.
Of all the Istari Gandalf is counted the greatest, for by his wisdom the free peoples of Middle-earth were guided to victory over the Dark Lord Sauron, who wished to enslave them. In this Gandalf was aided by Narya, the Elven-ring of fire, that Cirdan, lord of the Grey Havens, gave him, for Narya had power to make Men brave and resolute. By Gandalf's instigation Smaug the Dragon was slain and the Battle of Five Armies, the Hornburg and Pelennor Fields were won. By Gandalf's hand alone the Balrog of Moria was destroyed. Yet his greatest deed of all was his discovery of the One Ring and his guiding the Ringbearer to the place of its destruction. By this action the Ring was unmade, and Sauron and all his servants and all his kingdoms were brought to utter ruin. Gandalf's task upon Middle-earth was completed by this one act and so the Third Age ended with Gandalf's departure to the Undying Lands.
Second named of the Istari is Radagast the Brown, who lived in Rhosgobel in the Vales of Anduin. Radagast played a part in the White Council, which was formed to stand against Sauron, but it seemed his greatest concern was with the Kelvar and Olvar of Middle-earth and little of him is told in the chronicles of that time. He was wiser than any Man in the concernings of herbs and beasts. It is said he spoke the many tongues of birds. Even the Beornings and Woodmen of Mirkwood and the mighty Ent guardians of the Forest of Fangorn spoke with reverence of the wisdom of Radagast the Brown, for in the forest-lore he had no equal.
Last named of the Istari is Saruman the White, whom the Elves called Curunír, "Man of skill". When the Istari were formed, Saruman was counted the greatest of the Order. For many centuries Saruman wandered the lands of Middle-earth and eagerly sought to destroy Sauron the Dark Lord, but after a time he grew proud and desired power for himself. In the year 2759 Saruman came to Isengard, and Beren, the ruling Steward of Gondor, granted him the key to the tower of Orthanc, for it was thought that the Istari would aid the Men of Gondor and the Rohirrim in the war against the Orcs, Easterlings and Dunlendings. However Saruman made a mighty place of evil power there and summoned Orc legions and Uruk-Hai, half-orcs and Dunlandings to him. In Isengard he flew the standard of his tyranny, the black banner marked with a ghostly white hand. In this pride he grew foolish, until he was ensnared by Sauron, who commanded sorcery far greater than his own.
So the greatest of the Istari who had come to destroy the Dark Lord became one of his agents. Yet Saruman's power was annihilated by the wrath of the Ents, the valor of the Rohirrim and Huorns, and the wisdom of Gandalf. Isengard was destroyed by the Ents, his army was exterminated by the Rohirrim and the Huorns, and his staff was broken and his sorcerous power was taken by Gandalf. So low did Saruman fall that in his defeat he looked for petty vengeance in the tiny realm of the Shire, where the Hobbits, the least of his enemies, resided. Here in a pathetic bid for domination Saruman was bested by the Hobbits and slain by his own servant, Gríma Wormtoungue. When Saruman died, his body shriveled to a form without flesh. It swiftly became skin, skull and bones wrapped in a ragged cloak, and from it rose a gray mist in a column. For a moment, it is said, this gray form of Saruman's Maia spirit stood over his mortal remains, but a wind came and it vanished.