In reality, it was a war that had begun much earlier. If the Celestial had won that spectacular battle in the Wildlands, the fighting would have broken out immediately.
But he had lost that showdown, and then met defeat again on the bone galleon. These successive losses ate away at the Celestial's patience. And it was made worse by the fact that Big Fatty Zhang and some other lower-level cultivators had actually left the world, while he remained behind, stuck.
"Gravekeeper, you bastard, I know you have even more backup plans. But I'm done playing games!" Eyes flickering with madness, the Celestial issued his Dharmic decree!
The Wildlands were to be destroyed! The gravekeeper was to be killed! The Hell-Emperor was to be executed!
Perhaps it would destroy the heavens and crush the earth. Perhaps it would end the world. But at least the Celestial would finally become the true ruler of it all, and thus achieve his long-cherished wish of opening the Worldgate.