1: Alden is in the cage with the rest of the Humans, and not with Hallvard, right? It was a bit confusing in the post.
2: Can he change out of his disguise? And what abilities would he have?
1. Yes, he's in the cage with the humans. Thought I clarified that; sorry. Might've only done that in my outline and forgot to put it in the final post.
2. Demon-kin can change into, and out of, their human disguise at will. It's not painful at all, either. Maintaining the human disguise takes a small, but constant, amount of concentration.
As a demon-kin, you have innate demonic magic; for more info on their powers in general (including their human form), see this little section on the wiki. If you haven't already, of course. You also have some abilities based on the fact that you're a demon of Vengeance, such as a curse you can put on individuals to lower their defenses (i.e. dice rolls) against you for a time, as you enact your vengeance upon them.
Your demonic magic can vary; your most powerful magic would simply be soul related, as most black magic is: harming another's soul and weakening them, drawing another's life force into your own body, and minor necromancy spells.
Using any of your demonic abilities, however, draws you closer to your demonic corruption and increases your urges, as this magic is evil in nature. As for your true form, it is as most demonkin of Vengeance are: pale skin; bright, frost blue eyes; and black feathered wings.