In general, it seems like you need to create another NCF creature.
Together with the demons, the kraken won't look very good visually.
Dark elves often appeared (in both Heroes 5 and Heroes 7, as far as I understand) as cultists of chaos. So, I think the message will be clear. Plus, you can add to the boss's description: "cults of chaos consisting of various peoples of Ashan worship him," so it's clear that it's not just about demons.