In the movie, the Witch-king breaks Gandalf's staff. How did this happen and why (since it's not in the book)?
While Gandalf faces the Witch-king in Minas Tirith, there is no record of Gandalf's staff being broken. The flaming sword is seen in the scene in the book, from Return of the King, "The Siege of Gondor":
'You cannot enter here,' said Gandalf, and the huge shadow halted. 'Go back to the abyss prepared for you! Go back! Fall into the nothingness that awaits you and your Master. Go!' The Black Rider flung back his hood, and behold! he had a kingly crown; and yet upon no head visible was it set. The red fires shone between it and the mantled shoulders vast and dark. From a mouth unseen there came a deadly laughter. 'Old fool!' he said. 'Old fool! This is my hour. Do you not know Death when you see it? Die now and curse in vain!' And with that he lifted high his sword and flames ran down the blade. Gandalf did not move. And in that very moment, away behind in some courtyard of the City, a cock crowed. Shrill and clear he crowed, recking nothing of wizardry or war, welcoming only the morning that in the sky far above the shadows of death was coming with the dawn.
After that scene, Rohan arrives, and the Witch-king flies off to handle them.
The Witch-king's ability to destroy others' weapons is found in the books, however. From The Fellowship of the Ring, "Flight to the Ford":
Then the leader, who was now half across the Ford, stood up menacing in his stirrups, and raised up his hand. Frodo was stricken dumb. He felt his tongue cleave to his mouth, and his heart labouring. His sword broke and fell out of his shaking hand. The elf-horse reared and snorted. The foremost of the black horses had almost set foot upon the shore.
From the same chapter, after the wraiths are driven off, Aragorn is examining the cloak left by the Witch-king:
"Look!" he cried; and stooping he lifted from the ground a black cloak that had lain there hidden by the darkness. A foot above the lower hem there was a slash. "This was the stroke of Frodo's sword," he said. "The only hurt that it did to his enemy, I fear; for it is unharmed, but all blades perish that pierce that dreadful King."
It's likely that Jackson interpreted Gandalf's raised staff as a "blade to pierce" the Witch-king. It doesn't say in the books whether the Witch-king could destroy an Istar's staff, but I think that scene is Jackson's interpretation of these scenes in the book.
As far as I know, there was no Jackson commentary on that specific scene, so one can only speculate.
As for "why", the most cogent speculation I have seen on the forums is that the scene served to:
Highlight the Witch-king's power
Therefore make Merry's and Éowyn's takedown of him all the more heroic.
A second theory was that this was to highlight the power and contribution of the World of Men - Gandalf is almost beaten and can be killed by such a powerful adversary, yet saved by the fact that Witch-king needs to go away and deal with Rohirrim - which, if you know the backstory of Istari, actually jibes with Tolkien's overall vision of the order of things (Olórin's mission is to inspire Middle-earthers to fight Sauron, not to fight him himself with his Maiar powers).
This has further symbolic significance in that this mirrors Gandalf the White's breaking of Saruman's staff.
As for "how", there just isn't any info. The Witch-king holds a flaming sword, stares at Gandalf, and the staff explodes.
The short answer is, Peter Jackson thought it would be cool.
Gandalf is far more powerful than the Witch-king for a number of reasons.
- Gandalf was created at the literal beginning of time, while the Witch-king has been around for about an age.
- Gandalf is a Maiar who has been sent to Middle-earth, with reduced powers, the Witch-king is a human wraith with some of Sauron's power.
- Gandalf bears Narya, the elvish ring of fire, the Witch-king used to carry one of the nine human rings (Sauron took them back)
- Gandalf fought an unknown number of Nazgûl at Weathertop back when he was Gandalf the Grey (it is entirely possible that he fought all nine) and has only become more powerful.
- He defeated a Balrog (another Maiar) and then became even more powerful.
As cool as the scene may look, it has no basis in the world of the books. If the Witch-king was able to overpower Gandalf as easily as he did, there is literally no way that Éowyn and Merry would be able to touch him, let alone kill him.
I think it is important to keep in mind that Gandalf the Grey did not operate in a mode of "If it kills me, I will come back." His sacrifice at Khazad-dûm was absolutely done at risk and his arrival in Fangorn was very much a shock to everyone, himself included. You see in his manner of speech, not "I came back, I can do that you know." but "I have been sent back" and "I am Saruman as he was meant to be." It clearly came across to me that he was baffled by the turn of events, but pleased with the second chance.
I don't have a reference handy but I was told by a Scholar of Arda much greater than me that once the Nine had been fully subsumed by the power of Sauron that he took their rings back. This is why when he falls they don't have a ring of power in the pile of Nazgûl detritus.
Back to the original topic, there is no literary reason in the narrative for the Witch-king to shatter Gandalf's staff. It is in the movie for two intertwined reasons:
Peter Jackson likes to repeat himself (the Breaking of Saruman is cool, let's use that again. See also Hobbit2-Kili-Tauriel-OrcPoison vs. Fellowship-ArwennotGlorfindel-Frodo-MorgulBlade)
I think that it is not too hard to explain how the Witch-king did it: using his already known power of destroying enemies' weapons. The point is to know if he could do what he has done or not. I think not.
In the chapter "The White Rider", Gandalf says:
'Dangerous!' cried Gandalf. 'And so am I, very dangerous: more dangerous than anything you will ever meet, unless you are brought alive before the seat of the Dark Lord'. (p. 499)
So, one could easily say that the Witch-king is less powerful than Gandalf because only Sauron himself could defeat the White Rider. From that, the answer is clear: there is no situation with could get the Witch-king the power to destroy Gandalf's staff. It is a mistake Peter Jackson made in order to give much more glory to Merry and Éowyn.
In my opinion this is a quite clear indication of how Gandalf feels about the Witch-king's power.
“My friends,' he said, 'and all you people of this city and of the Western lands! Things of great sorrow and renown have come to pass. Shall we weep or be glad? Beyond hope the Captain of our foes has been destroyed, and you have heard the echo of his last despair. But he has not gone without woe and bitter loss. And that I might have averted but for the madness of Denethor.”
Plot reasons is what stands out to me. I'm pretty sure Gandalf could have bodied the Witch-king. He wasn’t "allowed" because of the plan to have Merry and Éowyn destroy the Witch-king. Peter Jackson also used Gandalf's staff being destroyed to emphasize the Witch-king's power. This way, it shows the great courage Merry and Éowyn had to face him.
Before the battle Théoden had said both of them had no place in battle and Éomer says something similar adding that they don't have courage like men. Further showing Peter's message that power is not what makes a person "strong" but courage, determination and compassion.
Gandalf's power was greatest when it was needed to guide and protect, the narrative always indicated that although his role is vital to the survival of the free peoples, his fate was never to overcome the dark adversary(s) himself. Not only that but, it's stated very early on that the fate (and power) of the Nine is bound to Sauron, so it would follow that the Witch-king's power was at its apex at the siege of Minas Tirith, when Sauron's might (albeit greatly diminished without the Ring) was at its greatest since his fall at the hands of Isildur.
My interpretation is that the destruction of Gandalf's staff by the Witch-king fits entirely to the narrative. The most common theme to the storytelling in both The Lord of the Rings and The Hobbit is that 'small is not necessarily weaker than big', so Peter Jackson's dramatic attempt to emphasise the strength of will of Merry, Éowyn and Sam and Frodo is not misplaced.
The scene of the Witch-king breaking Gandalf's staff seemed always odd and inconsistent with the text of Tolkien, as clearly pointed out by a citation before. It makes even less sense when one considers that Gandalf is now the highest member of the Wizard order and carrying the third Elven ring of power as it is revealed later on (at least in the books as I remember it). His power must be on the same level as Elrond's and Galadriel's, thus, superior to the power of the nine rings of men.
Peter Jackson probably wanted to increase the impact of the scene, where the Witch-king on his Nazgûl attacks Theoden and claims that no living man can kill him, but then Éowyn reveals herself as woman and destroys him. This resembles a crucial story of the Mahabharata, in which the invincible warrior Bhisma could only be defeated by someone who was not born as a man, Shikhandi/ni.
I think the reason the staff breaks is because
Sauron has infused a good amount of his power into the ring that the Witch King bears, and since Sauron is much much much more powerful than Gandalf, he could have given the Witch-king enough power to be more powerful than a normal Miar.
The staff is only a part of Gandalf's power, and Gandalf is not allowed to use his full power while on Middle-earth, only a small fraction of it. If he could use his full power, he could have destroyed Smaug, instead of getting a bunch of Dwarves to do it. The Balrog that Gandalf killed was probably much weaker than a Sauron-aided/power-induced Witch-king.
Therefore, for these reasons, mainly because the Witch-king was heavily power-fuelled by Sauron, it is very likely that the Witch-king can hurt Gandalf. But who becomes the victor in a one-on-one battle remains to be seen.