Whenever the time comes, we look for a human who'd be compatible with this power and fuse it together with their soul.

And thus is born—

The Child of Miare...

Yes. Although, even if it's just an embodiment of her power, it's also a part of Are's soul.

It and another soul can never meld together perfectly.

So is that the reason she collapsed?

Just so. What happened back there wasn't anything serious, but there will come a day that she can no longer live properly.