Gandalf the Grey had accompanied the Fellowship of the Ring, and Frodo the Ring-bearer, on his journey to Mount Doom. It has been a long, perilous and arduous journey for all of them, but especially for Frodo the Ring-Bearer, his responsibility was one of the most important thing ever : carrying the Ring, the Ring of Power made by Sauron. But he was a strong-willed Hobbit, he didn't succumb to the Ring's temptation. On their journey, they faced dark creatures such like Balrogs and etc, but others like Elves and Lord Elrond, were helping them on their dangerous quest. He, along with others, were almost near the bridge of Khazak-Dum. He had no power to help them magically, that would be defying the Istari directly. And if he did that, the Istari would take away his powers forever and he'd not be able to aid Frodo any longer. That was why he could only help them by solving riddles in lost and ancient languages and everything. He knew there were so many people and so many languages in Middle-Earth because the Creator had made them this way, it was his wish and all served a purpose in this life and place, like he, Mithrandir, as a angel in human form, served the Creator too. He was to aid and help Frodo the Ring-bearer much as possible to destroy the Ring of Power, forged by Sauron. This was his Mission, as Mithrandir the Grey on Middle-Earth.
He and the Fellowship of the Ring had walked days and days without foods nor fresh water, everybody was almost dying of thirst, but Frodo the Ring-Bearer had found a source. But he Gandalf the Grey or Mithrandir to Frodo, was suspicious of this source. The source was there on purpose, he thought, so he expressed his worries to Frodo :
"We can't stop here, Frodo. The source might be poisoned or there's something else in the water."
"How can you tell, Gandalf?"
"Just because it's on our road when we need water the most and we're in Dark Moria and it's one of those sources from Mount Zirakzigil."
"But Gandalf, if we don't stop here, men, hobbits and everyone in the Fellowship are going to die of thirst!" Merry exclaimed.
"Even if it were poisoned, I'd still drink from the source. I'm so thirsty I could dry a river!" exclaimed Frodo."
"I understand everyone is thirsty, but think about it. Why would the source be there, when we most need water? This could be a only a mirage."
"Gandalf, let us drink from the source. We've been walking for days without water, we can't hold any longer."
"If you all say so, but don't say I didn't warn you all of the source's possible dangers."
"Gandalf, I think you're just beginning to become a little too distrustful. What could there be in such a clear and fresh looking source? I want to drink from it, it's water is so appealing to me now."
"My last words : don't say I didn't warn you beforehand."
"We know, Mithrandir, but let's just stop and drink here. Everyone's mouths must be so dry by now."
Everyone, except he, Gandalf himself, drank from the source. Frodo, he saw took some deep long drinks, and everyone else did too. But he didn't want to linger there unnecessarily, so he urged the group to quickly finish drinking.
"All right, if everyone's done drinking, let's go. No one's stomach's hurt or anything like that?"
"No, Gandalf. I told you the source wasn't poisoned, you were too suspicious. You should have drank from it yourself."
"Prudence, Frodo. Prudence is much needed on quests such like ours."
"Can we rest here for the night, Gandalf?"
"Don't you want to go back to your Shire, as quickly as possible, Frodo and you Merry, Pippin?"
"Yes Gandalf, we want to. So we must hasten our pace?"
"Yes. Right ahead is Kazahd-Dum the Bridge. I wonder what we're going to face, Gandalf."
"I don't know, we'll see." said Gandalf mysteriously. But he had some fair idea about what was lurking on the Bridge's sides : Balrogs, an ancient race of evil, bred by Morgoth also, so they were Lord Morgoth's, the Lord before Sauron's minions. As they were crossing the Bridge now, they indeed encountered a Balrog, one of the worst evils possible in Middle-Earth, half-way to the other side of the Bridge. Since he alone knew how to fend these off, he was the only one who fought with it, and others watched as he fought it, with ancient magic known only to the Istari. With his staff, he made the Bridge crumble, and the Balrog fell from the Bridge of Kazhad-Dum, bringing him down with it. Others still watched him as if paralyzed, while he fell with the Balrog, he said :
"Fly, you fools!" And Frodo began to run, and one by one they followed the Ring-Bearer.
He drew one last breath before letting himself fall endlessly into the dark abyss below Kazhak-Dum. He fell, fell and fell with the Balrog, for a very long time, and they finally stopped falling. The next thing he knew, he was climbing to the top of Mount Zirakzigil, with the Balrog tailing him closely behind. At the top of Zirakzigil, he fought the wretched creature for what seemed like forever to him until neither had no more energy. He and the Balrog died of exhaustion and he felt his vitality gone from his body drop by drop. But just as he was dying, he heard someone from above saying to him :
"Mithrandir, your Mission on Middle-Earth isn't over yet! You still have your duties with the Fellowship of the Ring! So I am sending you back to Middle-Earth and thus you shall be reborn as Gandalf the White, instead of Gandalf the Grey."
"Who are you?"
"Just know me as your Creator, Mithrandir and I am sending you back because you haven't finished your task in life yet, Frodo the Ring-Bearer and the Fellowship still need you."
"Is that my purpose in life, my Lord? And why as Gandalf the White?"
"Yes it is and because Saruman has become Sauron's wizard, so I have cast him out of Istari and you shall take his place as Istari's Leader now. And come, Gwaihir, you shall carry Mithrandir the White to Lothlorien, the only place where there is still some Elixir of Angel left and where the Fellowship is right now."
Thus Gwaihir, commanded by the ultimate Creator of all, carried him to the Forest where the Elves dwelled. In Lothlorien, the White Lady Galadriel gave him said Elixir and he was reunited again with his young Hobbit friend Frodo and the Fellowship of the Ring.
"Gandalf, is that you?!" asked Frodo, flabbergasted with joy.
"I am not Gandalf the Grey, I am Gandalf the White now, Frodo, Ring-Bearer."
"Gandalf the Grey or Gandalf the White, no matter, you're still our Gandalf! And you must promise us to never leave us again, we'll need you when we'll face Sauron's army at the Black Gate, why, we thought you died fighting the Balrog!" His little Hobbit friends hugged him tight.
"Merry, yes I died, but I was brought to life again by the Creator and I promised him I would be there with you through the end of this all."
When he and the Fellowship were near the Black Gate, they weren't allowed entrance to Mount Doom, because the task was for Frodo the Ring-Bearer alone. So he and the others waited there, fearing for their heroic and dear friend's life. He saw the Dwarf was fidgeting there and pacing there and there and he hated inaction, so he asked the Fellowship :
"So what are we to do now? Just waiting for Frodo and Sam to return from Mount Doom? I can't wait any longer."
"Gimli, we can't do much for them, and I as much as you, am just as unsettled about them too."
"But, Gandalf, we can create a diversion, to deflect Sauron's eye on Frodo?"
"Yes, I agree with Aragorn, we could do at least this for them."
"Friends, friends, I know you're all worried about them, but we can't do anything right now for them. We'll be killing countless innocents in this reckless plan of yours, Aragorn King."
"But it's still better than to do nothing. And small chance of success, high risk of dying? I'm all for it!"
"So who's for Aragorn's idea and who's not? We're going to decide by a vote. If you're for this dangerous idea, raise your hand."
Naturally, Gimli's was first in the air, and one by one, all in the Fellowship and others like Lady Eowyn of Rohan raised their hand in favour of the diversion, except him.
"Fine, since I'm the only one against it, we'll be creating a diversion for Frodo and Sam."
Aragorn and Eowyn were the ones to organize the diversion and not much later, Elves, Dwarves, Men and many others volunteered for it. Contrary to what he feared for these volunteers, however, they successfully diverted Sauron's eye. Eventually it was Gwaihir, the Lord of all Eagles again on the Creator's orders, who flew Frodo from Mount Doom down to the Black Gate. He, as Mythrandir, then suggested for Frodo as well as Sam to go the Undying Lands to heal. The Elves accepted his proposition gladly and allowed his two Hobbit friends to sail to the Undying Lands with them after the war of the Rings.