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I already answered in Pokemon General Thread topic, but I might as well answer here too just in case you missed it - it's Meloetta's Relic Song, though I'm unsure if it's Gen 5 or Gen 9 version. BGM volume way too small to know for sure, I even had to listen several times until I can at least recognize the tune.
(Seriously, what was even that guy's reason in putting a BGM there if the volume is that low???)
It's definitely not the Gen 9 version, since the video came out before Pokémon Scarlet and Violet. I also thought it was Relic Song at first, but no version of that I could find includes the faster part starting at 12:44.
Edited by rjd1922 Keet cleanup
~Shining Stardust: Not sure if pings work here, but it's not Relic Song despite the similar instrumentation.
Keet cleanup
What's the Pokémon music used in the background here?