There are secrets far and wide within The Legend of Zelda, but one of the more impressive ones might have just been uncovered. It turns out that the Guardian Battle theme you hear in Breath of the Wild is actually an inversion of Zelda's Lullaby from Ocarina of Time.
Over on the Breath of the Wild subreddit, one user managed to uncover that a part of the Guardian Battle theme uses the exact same note progressions as sections of Zelda's Lullaby, but flips the notes. So for example where both tracks play a B, Zelda's Lullaby progresses up a Minor Third to a D, whereas the Guardian Battle would progress down a Minor Fourth down to F sharp.
The original user noted that if you listen to the track just below at 21 seconds, you'll hear the piano play the same note from Zelda's Lullaby (starting with a high B). This is the same note that the Guardian Battle theme uses, which got the user thinking that there could be a connection between the two.
Next, the user aVeryLittleStoat realized that they're both played in three time, the tempo of the whole track (wow, my euphonium lessons are finally paying off). Lining up the two sections against one another, you can see that they're a reflection of one another, using the same note progressions in the same order, but going in the opposite directions.
Thematically, it's nothing short of genius by Breath of the Wild composers Manaka Kataoka, Yasuaki Iwata, and Hajime Wakai. Remember how the Guardians were partially conceived by Princess Zelda in Breath of the Wild, only for Calamity Ganon to corrupt them into his minions? The Guardian Battle theme as an inversion of Zelda's Lullaby is reflective of how Ganon corrupted them, and won them over to his side, so to speak.
This isn't actually the only time Breath of the Wild has riffed on Zelda's Lullaby. Further down on the same subreddit thread, user drewcomputer picked up on Breath of the Wild's Horse Riding theme also using those same three notes from Zelda's Lullaby, which you can hear in the video below at around the 40 second mark.
Honestly, a big congratulations is in order for aVeryLittleStoat. Even after listening to both tracks and reading through the users explanation, I've got absolutely no idea how they managed to identify the inverted note progressions in the first place. Hats off to their fantastic musical intuition.