Demonic possession.

At some point in your life you have died or been killed. You’re soul has been to hell…been corrupted by demons, tortured by them, until such time as your defence broke down and you let them in. 

Now your soul is back in its body…but what came back with you?

All your memories from before you died are gone, sometimes when you sleep or when you are frightened, you get flashbacks from your time in hell.

Upon you dropping to 0hp, roll a Constitution Saving throw, if you beat a 15 you go into death saves, if you fail, you roll a d10. The result of that roll picks the demon that possesses you (picked from a pre-organised selection). Every time the demon does anything (take an turn/action, is dealt any form of damage) you roll a d20. Only on the roll of 20 do you manage to retake possession of your body. Until then the demon is free to do what it wants.

It will have seen everything you have been doing from behind your eyes. It can choice to help out in whatever quest you are currently doing or it could turn on your party members. Either way it will always defend itself if attacked.

If the demon takes so much damage that it drops to 0HP then it must pass a Constitution Saving Throw (15) of its own or be vanquished back to hell. Either way you take back possession of you body, but are immediately in death saves.