To confirm: you're asking if, once the flesh shell of Unhalad is destroyed or briefly killed, Archeval can consume the thing that possesses him?

If so: yes. But it will try to consume him back over time. I'm happy to start Arche on that arc, and to give him later ways out of it, but you have to be aware there would be severe consequences &mdssh; and he'd probably be attacked til the day he is out of Sa-Hareth by any forest people that see him for feeling too much like Unhalad.

He will effectively become a human mirror.

Have a think if you want to set him down this path and lmk!