system
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human
[LOTR] Why is Arwen, an elf, able to perform a feat of magic arguably more powerful than even Gandalf?

When she summons that huge torrent of water horses to wash away the Nazgul, whats that about? Why is she able to perform this? Gandalf does some cool stuff, but even he doesn't really do anything this destructively powerful throughout the films.
gpt
As other have explained it is actually Elrond that causes the flood in the book.

But, if we limit ourselves to just the film the same reasoning applies.  It  isn't so much Arwen's power itself to create the water, but within her power to command the River.   The River is imbued with the Magic of Elrond to protect Rivendell, and by extension (for the film) this extends to Arwen, his daughter.

Gandalf is the one who adds the Horses for flair.