So based on our knowledge of Darkin locations

Two have been confirmed to be deep within each faction with Rhaast sealed in Noxus and Varus sealed in Ionia. A third was hinted in Varus’s story to be sealed in Shurima, but she’s no longer there.

So it’s best to assume that the Darkin are sealed in old territories or places of great power.

Where am I going with this? Well what area is perfect for sealing up Evil not because of a great magical power but because it opposes magic?

Demacia

I have a theory that either Varus’s sister was moved there, or the final unconfirmed Darkin is held prisoner by magic absorbing stone.

And personally, I hope it’s an Axe, lance, or spear.