I didn’t do a full demonic possession but did a cursed item of Lolth which was very fun. The way I did it was each crit forced them to roll a doom die that is it hit a 1 the die decreased. As the die decreased the power of the item increased. Moreover the downsides and secretly how difficult it was to remove increased. So it started with a 1d10 and went down down to 1d4. They quit at 1d8. At lower die they got powers that would cause a roll of a doom dice which they could use at will.
This allows the players to dig their own grave as they use it more. Also the corruption starts slow and then ramps up more until it’s a full fall. It was fun to give then agency.
I would give them a list of demonic powers some of which would give some corruption but they can use it as often as they like. Let them have agency as they corrupt themselves
I didn’t do a full demonic possession but did a cursed item of Lolth which was very fun. The way I did it was each crit forced them to roll a doom die that is it hit a 1 the die decreased. As the die decreased the power of the item increased. Moreover the downsides and secretly how difficult it was to remove increased. So it started with a 1d10 and went down down to 1d4. They quit at 1d8. At lower die they got powers that would cause a roll of a doom dice which they could use at will.
This allows the players to dig their own grave as they use it more. Also the corruption starts slow and then ramps up more until it’s a full fall. It was fun to give then agency.
I would give them a list of demonic powers some of which would give some corruption but they can use it as often as they like. Let them have agency as they corrupt themselves