It's part of the Churning Mists quest arc.
SPOILER WARNING
The moogles of the Twelveswood meet with the moogles of Moghome and discover that their ancient legends aren't so different, coming to the conclusion the Good King Moggle Mog XII and Chieftain Moggle were probably one in the same. Myths combined, Moggle would have been a great leader who, after fire rained from the sky and broke the flying continent into pieces, led a band of moogles away so that they might still live in harmony. The thing that connects the two groups ends up being the kupo nuts and trees, which were brought from the heavens to the grounds below.
The story has been fancied up over the years so you'll need to shave some of it off, what with the Twelve and the warring gods and the great king Moggle and all that. Good King Moggle Mog XII might still be more myth manifest than not, but there's shards of truth, it seems!
I, myself, am doing transcripts as I go, so it's been slowgoing. Someone who's seen the information first-hand instead of hearing about it from excited friends (who try their best not to spoil the scenario, at least) might be able to elaborate a bit more accurately.