You know, I love this notion of two black cowls also, but the one about the demon somehow "leaping" from person to person and orchestrating it all…that's the best!
If your players haven't already completed the Winds of Magic adventure, just make the demon Bobo(the little pet bird) and have him turn into that big demonic creature looking for a re-match. Man, that would be priceless.
Here's to Bobo! The Enemy Within(The BirdCage). lol.
If going with possessing daemon, or just using that anytime, I highly recommend checking out the Tome of Corruption (2nd edition) "Ritual of Exoricsm" (page 127).
This earlier edition ritual is ready-made to convert to a progress tracker with multiple stages and rolls, with differences in the steps to keep it interesting, that can easily make an entire "Act" (and give you a very Linda Blair "exorcist" type of scene to boot).