Character and Background development

[Shadowbane13]

So I came up with a somewhat cool, or at least i think cool, and different concept for the background "The haunted one". Instead of being basically haunted by ghosts, or past terrors, or basically PTSD, I came up with an idea of that at one point (prior to session 0) that my character had actually died. He was resurrected but when he was it was not just his spirit that entered the body. I said that a demon, or some sort of evil spirit attached itself to him and now he has an interior fight with it and trying to prevent it from coming out. My DM decided that at each long rest, when my body is least likely to fend it off I would have to make Con saving throws. If I fail more than 3 times in a row, the demon takes over my body. The real question here from me is this:

How should I play out the demon? I consider him evil but I don't want to outright just attack and break things. I'd probably get leveled by my party anyways. I was thinking I'd see what Giant thinks of this idea.

[Sparky McDibben]

So there's a few different ways to go here, but all of them depend on what the demon wants.

Assuming that the demon's first priority is to stay out of the Abyss (everyone wears sandals with socks down there, it's terrible), I would argue it would seek some sort of fallback in case the current meatbag (that's you) gets destroyed. This might include a specially prepared ritual site (talk to the DM), a magic jar (for it to hide in), etc. Second, it would set about providing for itself, making sure to acquire wealth, land, and the services of morally ambiguous mercenaries to fend off paladins.

However, assuming that the demon wants to bring the Abyss here...well, then things get interesting. You might start trying to found cults, making deals with people to give them power or wealth, and feeding on their pact with you. Again, talk to your DM. I wouldn't allow this to do anything more to your character than give them maybe a luck point, but you'd essentially start "banking" the stored energy to eventually open an Abyssal gateway. Other options might include trying to foster an alliance with an alkilith (from Mordenkainen's), learning the secret rite to open it, etc. And you'd have to do all this while hiding your true plan from your party.

Alternatively, your DM might take a middle ground and rule that the demon will only derive so much sustenance from your soul, requiring it to found cults to feed on their worship. That gives you a nice RP angle, and a goal for your character not directly tied to the story.

As far as RPing the demon, I'd say it tries to be like your character for as long as possible, but maybe gets real uncomfortable in churches or around paladins. It's not that they cause the demon pain - it's that they remind it of having divine grace felt like, back before the Fall. And they remind it that forgiveness is open it, if only it repents.

That might open up a neat redemption arc for the demon, too, depending on your DM.