A theory about Galactus, The Chronoserpant, and the Battlerealm itself
I'm not quite sure abut the story, so maybe this theory isnt even in the direction of being true, but:
So we thought Galactus is the bad guy, that he will be the antagonist of Act 8, but in that video about who killed the Maestro, where The Chronoserpant was introduced, we saw Galactus fighting him.
It was mentioned that The Chronoserpant is a Celestial parasite, I guess that means he lives on Celestials, drains power from them as he gets stronger, like what parasites are for us.
It is known that the battlerealm was created from a dead body of a Celestial, maybe that's why The Chronoserpant is here? He uses his body as the host? Not sure if that makes sense as the Celestial is dead, but it might!
So maybe the reason Galactus want to devour the Battlerealm, is to get rid of the parasite within it? Hench why they fought in that video.