Thought of the same exact thing. The peak of Mt. Targon clearly connects to space and some otherworldly realms within. It could even connect to both Celestials and the Void at once.
Which would explain the waves of bad monsters and singular good monsters who cannot be the same species.
The stories of the Lunari hint the darklings have eaten through countless worlds before. It would be absolutely amazing if it was really the Void devouring various realms - and that the celestials, who were damaged by these calamities, are now warning the humans of the incoming swarm.
It could mean an entire separate war on a brand new, epic scale! And a sense of everyone being completely screwed on a brand new, epic scale!
Really, a Celestial - Void interaction is something I wanted to see forever. Be it a truce, be it a war, I'd gladly take it in any shape or form.