In J.R.R. Tolkien's world of Arda, The Istari(plural of Istar) were five (or maybe more) beings sent to Middle- Earth by the Valar to unite and counsel the Free Peoples in their struggles against Sauron. They were forbidden to dominate the Free Peoples or to match Sauron's power in order to defeat him themselves. Saruman disobeyed this order and aided Sauron and built an army of Orcs in Isengard. Gandalf the Grey, who had become Gandalf the White, cast him from the order, Saruman was later slain in The Shire, which he had taken over and nearly destroyed. At the end of the third age the job of the Istari was done and they passed from sight, Gandalf was a ring bearer and went to Valinor, which is probably where the others went to as well, though they were not ring bearers.

Tolkien said that they were Valar "of a sort”. They had the sprit of Maiar, though the had the frail form of an old man, however they aged very slowly and were very vigorous. They had great skill of both body and mind, and were very powerful wizards. Their powers were focused through their staffs. By the end of the Third Age Gandalf the White was arguably the most powerful being in Middle- Earth.

Each member of the Istari had different colour robes to distinguish their power within the order.

Saruman the White- Leader of the order, until Gandalf cast him out.
Gandalf the Grey- the second most powerful in the order, until he cast out Saruman. He became Gandalf the White when he slew the Balrog.
Radagast the Brown- third most powerful in the order. Saruman deceived him into getting Gandalf captured in Isengard. He was a loved of animals, especially birds.
There were also several other Blue Wizards, who lived in the East, however nothing is recorded of these people.

A note by Tolkien (in Unfinished Tales), dated by Christopher Tolkien (his son) as probably coming from 1972, basically states right out their origins and purpose.

We must assume that they (The Istari) were all Maiar, that is persons of the 'angelic' order, though not necessarily of the same rank. The Maiar were 'spirits', but capable of self-incarnation, and could take 'humane' (especially Elvish) forms... Now these Maiar were sent by the Valar at a crucial moment in the history of Middle Earth to enhance the resistance of the Elves of the West, waning in power, and the Men of the West, greatly outnumbered by those of the East and South. It may be seen that they were free each to do what they could in this mission; that they were not commanded or supposed to act together... and that each had different powers and... were chosen by the Valar with this in mind.

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