Kalye/Morgana Lore Speculation
From all of the evidence at hand, I assume Kayle and Morgana are each wielding a Darkin. Not 2 Darkins, but 1 Darkin between them.
It is why Morgana is stuck as a human, and why Kayle can't reach her full potential, despite Morgana having taken over her host and Kayle completely dominating her weapon.
The broken weapon being a scale, like this. https://orig00.deviantart.net/40e9/f/2009/327/1/8/tattoo_for_kudlaty__sword_by_lws1988.jpg
Now here's the fun part. Like we've seen with Varus, all Kayle and Morgana would have to do is touch the sword and chains back together and BAM! They fuse into a Darkin. Kayle reaches her full potential and Morgana becomes free from her flesh and Runeterra.