In the development page right now, there's one feature that really caught my eye.
Once the new wizards are all developed, they will be capable of giving the player different quests/tasks/whatever, and one of these "quests" is to be sacrificed to become the "vessel" of some demon. I'm really curious what that would look like. The obvious answer would be that you're sacrificed, the demon takes over your body, and you no longer play that character... But that's so boring. What if you retained control of the character, but now, your character had traded some free will for the demon's powers?
For instance, just the first, easiest example I could think of, if one of your companions says something negative about the demon's values, or the demon himself, perhaps the demon takes over for a second, and initiates combat, before allowing you to finish that combat? Any attempts to surrender, or ask for a cease in hostilities are not allowed by the demon, and the demon only lets you leave that area once your companion is dead.
But perhaps this could go further as well. For instance, maybe the demon was sealed away in the first place by some elf. And once in a while, your demon friend could take over during conversations, and ask random NPCs where his elven enemy is, trying to find specific directions. Once specific directions were found, perhaps the demon gives you the task of going to find this elf. If the player doesn't head to the specific tile pointed out to them by whatever NPC, then eventually, whenever the player enters fast travel mode, the demon will simply take over, and force the player down the quickest path to the destination, stopping only to allow the player to take care of their bodily needs, like sleep and food.
Maybe the demon could act in self interest, too. Like, for instance, if you're losing a fight, maybe the demon pulls out whatever magic he has in his back pocket, in a last ditch attempt to save your life, and takes over your body to force you to run away.
Better yet, though, would be if all of these things depended on the demon's randomly generated values and characteristics. Like maybe, in the case of killing your companion, should the demon dislike violence, it might keep the fight as a brawl, and prohibit all attempts by the player to go lethal, only going lethal if the companion does first. Maybe, if the demon believes in harmony, he might ask you your values, and attempt to find a middle ground between your beliefs and his. Or, if he doesn't believe in harmony, but does want power, he could ask you your values, just like the harmonious demon, but maybe he somehow punishes you for disagreeing with his values.
All in all, a demon system that makes it very clear to the player that they're in the demon's head, rather than the other way around, would be freaking awesome! And I'm curious, what other scenarios could a demon's vessel cause? Like, how might the demon influence the daily life of the hero?