The Silmarillion

Then the name of the forest was changed and Mirkwood it was called, for the nightshade lay deep there, and few dared to pass through, save only in the north where Thranduil’s people still held the evil at bay. Whence it came few could tell, and it was long ere even the Wise could discover it. It was the Shadow of Sauron and the sign of his return. For coming out of the wastes of the East he took up his abode in the south of the forest, and slowly he grew and took shape there again; in a dark hill he made his dwelling and wrought there his sorcery, and all folk feared the Sorcerer of Dol Guldur, and yet they knew not at first how great was their peril.

Even as the first shadows were felt in Mirkwood there appeared in the west of Middle-earth the Istari, whom Men called the Wizards. None knew at that time whence they were, save Círdan of the Havens, and only to Elrond and to Galadriel did he reveal that they came over the Sea. But afterwards it was said among the Elves that they were messengers sent by the Lords of the West to contest the power of Sauron, if he should arise again, and to move Elves and Men and all living things of good will to valiant deeds. In the likeness of Men they appeared, old but vigorous, and they changed little with the years, and aged but slowly, though great cares lay on them; great wisdom they had, and many powers of mind and hand. Long they journeyed far and wide among Elves and Men, and held converse also with beasts and with birds; and the peoples of Middle-earth gave to them many names, for their true names they did not reveal. Chief among them were those whom the Elves called Mithrandir and Curunír, but Men in the North named Gandalf and Saruman. Of these Curunír was the eldest and came first, and after him came Mithrandir and Radagast, and others of the Istari who went into the east of Middle-earth, and do not come into these tales. Radagast was the friend of all beasts and birds; but Curunír went most among Men, and he was subtle in speech and skilled in all the devices of smith-craft. Mithrandir was closest in counsel with Elrond and the Elves. He wandered far in the North and West and made never in any land any lasting abode; but Curunír journeyed into the East, and when he returned he dwelt at Orthanc in the Ring of Isengard, which the Númenóreans made in the days of their power.

Ever most vigilant was Mithrandir, and he it was that most doubted the darkness in Mirkwood, for though many deemed that it was wrought by the Ringwraiths, he feared that it was indeed the first shadow of Sauron returning; and he went to Dol Guldur, and the Sorcerer fled from him, and there was a watchful peace for a long while. But at length the Shadow returned and its power increased; and in that time was first made the Council of the Wise that is called the White Council, and therein were Elrond and Galadriel and Círdan, and other lords of the Eldar, and with them were Mithrandir and Curunír. And Curunír (that was Saruman the White) was chosen to be their chief, for he had most studied the devices of Sauron of old. Galadriel indeed had wished that Mithrandir should be the Lead of the Council, and Saruman begrudged them that, for his pride and desire of mastery was grown great; but Mithrandir refused the office, since he would have no ties and no allegiance, save to those who sent him, and he would abide in no place nor be subject to any summons. But Saruman now began to study the lore of the Rings of Power, their making and their history.

Now the Shadow grew ever greater, and the hearts of Elrond and Mithrandir darkened. Therefore on a time Mithrandir at great peril went again to Dol Guldur and the pits of the Sorcerer, and he discovered the truth of his fears, and escaped. And returning to Elrond he said:

‘True, alas, is our guess. This is not one of the Úlairi, as many have long supposed. It is Sauron himself who has taken shape again and now grows apace; and he is gathering again all the Rings to his hand; and he seeks ever for news of the One, and of the Heirs of Isildur, if they live still on earth.’

And Elrond answered: ‘In the hour that Isildur took the Ring and would not surrender it, this doom was wrought, that Sauron should return.’

‘Yet the One was lost,' said Mithrandir, ‘and while it still lies hid, we can master the Enemy, if we gather our strength and tarry not too long.'

Then the White Council was summoned; and Mithrandir urged them to swift deeds, but Curunír spoke against him, and counselled them to wait yet and to watch.

‘For I believe not,’ said he, ‘that the One will ever be found again in Middle-earth. Into Anduin it fell, and long ago, I deem, it was rolled to the Sea. There it shall lie until the end, when all this world is broken and the deeps are removed.'

Therefore naught was done at that time, though Elrond's heart misgave him, and he said to Mithrandir: