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 Seige 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 other's 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 WK could destroy an Istari's staff, but I think that scene is Jackson's interpretation of these scenes in the book.
AFAIK, 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 Witch-King's power
Therefore make Merry's and Eowyn'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 WK 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 (Olorin's mission is to inspire Middle Earthers to fight Sauron, not to fight him himself with his Mayar 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. WK 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 Nazgul 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 Eowyn 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-Dum 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 dont have a ring of power in the pile of Nazgul 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:
1 Dramatic Effect
2 Peter Jackson likes to repeat himself (the Breaking of Saruman is cool, lets use that again. see also Hobbit2-Kili-Tauriel-OrcPoison vs. Fellowship-Arwen*notGlorfindel*-Frodo-MorgulBlade)
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 Galdriel´s, thus, superior to the power of the 9 rings of men. Probably, Jackson wanted to increase the impact of the scene, where the witch king on his nazgul attacks Theoden and claims that no living man can kill him, but then Eowyn 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.
In my opinion this is a quite clear indication of how Gandalf feels about the Witch Kings 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.”
I think the reason the staff breaks is because,
A. 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.
B. 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 remainsto be seen.