But seeing as they've been doing this for about a thousand years and satan just appeared some time ago, that couldn't be the reason

Maybe some of the big shots of the Undead knew that Satan would come back eventually or even that he hid in the Tree of Life.

I have a feeling that all this is tied to Ouyang. Qian called him a half breed before and I think he's that half god half demon that Xiu mentioned before. That also makes me think he is actually the reason for the war to begin with and if that's the case, he's been orchestrating absolutely everything, from the warlocks experimenting on people, to the Broken Star Palace transferring dou qi and magic, to helping Long Yang obtain his position and helping the undead build their army. After all Ouyang was the one to lure Long Huang to Radiance Continent, and that might have been to help Long Yan. Ouyang might have helping Satan all along.

@Sassysosa I mostly agree with you. But, I'm not sure if he really the reason for the war between gods and devils thousands years ago. The two races has always been in conflict even before the couple of god and devil has their mixed-race child. I can only imagine the discrimination he had received as half-breed--both from the gods and the devils--has twisted his character and outlook of the world. I'm also not sure if he was on Satan's side rn. It's more like he wanted to plunge the world and all the races into chaos as a form of his revenge.