Like, the description just says that it’s liquid you can find in the Dark Sea. We have no idea why you get poisoned when you get in, but it can be used to brew burning potions (just what).
Also it seems that liquid starlight pools tend to be bigger the further you go, so it’s probably linked to magic pollution in the Dark Sea. However, I have another theory.
It could be linked to the origin of the Milky Way in Greek Mythology, which is basically:
Heracles is born
Zeus wants him to be fed with Hera, uh, breast milk (I don’t how to blurry the text sorry)
Knowing she won’t accept it, he does that when she’s sleeping
She wakes up
She moves Heracles away
Her milk goes into the sky, creating the Milky Way
(Yeah Greek Mythology is really weird sometimes)
So Liquid Starlight could be milk that didn’t get in the sky and fell back on earth, or milk from the Milky Way falling on earth after the “creation” of the Dark Sea, and since the Dark Sea covers the vast majority of the earth, that’s why you can only find it here.
(Why did I make an entire theory on a potion reagent wtf)
I am almost completely certain that it’s just a reference to Liquid Starlight from the Astral Sorcery Minecraft mod. May not have any real in-universe explanation.