Wait wait wait! What the heck just happened?

End of the world?
I guess in this manga, when they can't just regenerate, Hourai beings were reincarnating. It's a notion that appears every now and then.
And the only way for Kaguya to reincarnate was to be born by Mokou.

Still trying to figure out what the deal with Eirin is there. It seems to imply that she isn't actually a living being. Certainly it's clear that she can't carry a child for some reason.

Unfortunately, there's nothing in the main story that sheds any light on that either, even if it does explain that Kaguya reincarnates rather than regenerates under some circumstances.

Still trying to figure out what the deal with Eirin is there. It seems to imply that she isn't actually a living being. Certainly it's clear that she can't carry a child for some reason.

Unfortunately, there's nothing in the main story that sheds any light on that either, even if it does explain that Kaguya reincarnates rather than regenerates under some circumstances.

Not everything in this world is meant to make sense...

Things in stories are meant to, as a general rule.