I have remembered a middle-grade book (series?) that I'd read probably around 4th-6th grade. These are all the details I can remember:
It was a medieval fantasy esque setting.
It featured a world with some sort of king or dark lord that the protagonist was trying to defeat - for some reason, I don't remember why, this king was almost undefeatable. He could only be defeated by a secret word.
Parts of the secret word were scattered around the land in syllables.
The book begun with an ominous prologue in which a character had finally gathered all the parts of the word, and confronted the king - I think he might've been captured or something like that, and was brought before the king. When he said the word, it didn't work. I don't remember perfectly, but it might've been because the 'king' was an imposter, staged there to make the person say it and then the real king would come out.
After saying the word, the person who said it immediately forgets it.
After arriving at the blind king's decrepit keep, Jason learns that
the "blind king" is really Galloran, a broken hero who already tried
and failed on the same quest that Jason is now committed to. A few
moments after learning of Galloran's true identity, Jason is
introduced to Rachel, a girl his age from earth, who arrived in Lyrian
by walking through a stone arch. Galloran then tells them that they
are central to a prophecy spoken by a powerful seer, one in which they
will destroy the tyrannical wizard emperor, Maldor. Galloran points
out that the easiest (though still extremely difficult) way to
accomplish this is to seek a Word of power that when spoken in
Maldor's presence will theoretically destroy him. There are six
syllables to The Word, the first of which had already been discovered
by Jason at the repository of learning. Each syllable of The Word is
protected by a guardian, each of whom will only reveal their syllable
to those embarked on the quest to destroy Maldor. However, some
syllables have also been written down (such as the one at the
repository of learning), removing the necessity of directly seeking
out the guardians of those syllables. They may only speak or write The
Word in its entirety once, after which the individual that did so will
forget it. Since Jason was the only one to read the book containing
this information, Rachel may opt out. However, she decides to join
him. Satisfied with their decision, Galloran presents Jason with a
dagger whose blade shoots out of the handle at the pressing of a
button on the pommel, while Rachel is given an orantium sphere, a type
of grenade with an explosive crystal encased in glass that explodes
when exposed to air or water. After being given a starting point by
Galloran, Jason and Rachel embark on a very long quest for The Word.