>The empty sky broke asunder,
>a hole appearing where there had been none.
Ultra Wormhole
>A single beast appeared from in it:
>it was named the beast that calls the moon
Assuming it's Lunala, but it's vague. Calling the moon could also mean creating an eclipse.
>The king of Alola bowed before it:
> the beast that stole all heavens' light.
The humans immediately welcomed it...
>The island guardians fought against it,
>but in the end they lost the fight.
...but this implies the Tapus wanted it gone and lost. Not necessarily that they lost the actual fight, but since the humans decided they wanted it to stay, the Tapus' goal of driving it off was what was a failure.
>Then did the beast that brings the dark
>cast its pall on the line of kings.
It seems that the age of kings was put to an end by the Ultra Beast that appeared. The Tapus supposedly chose their kings back then, and there's a book about the Tapus abandoning the kings in the same library. Was this event the one that caused the Tapus to abandon the people? In addition, "pall" is a death shroud, or covering something with darkness.
>So would the beast that had won
>mark the path for all such finished things.
>Beast of sun and beast of moon.
Obviously Solgaleo and Lunala here.
>Through their union, they brought new life.
>A fragile heir in Alola born
Cosmog
>that island guardians would keep from strife.
The Tapus were made guardians over Cosmog and basically told to keep it from humans.
>The ancient kings sang their thanks
>for Lunala with song of flute.
Why? What exactly did Lunala do except cause the Tapus to abandon the kings and bring darkness to their kingdom, unless it wasn't Lunala/Solgaleo?
>Two tones rang out across the altar--
>a perfect pair, ever after mute
The flutes were made ineffective without a Cosmoem present.
The tale seems to be about Lunala/Solgaleo, but it's off. It's more likely the story is about Necrozma, who only appeared after Lunala/Solgaleo showed up once again.