A king so vile and treacherous was cursed many centuries ago to forever roam the earth. He is now a serf of earths will and shall be anywhere needed to stop the destruction of earths its bueatiful landscapes and creatures.
The PCs find a strange helmet, when donned it will take control of that PC and speak and act trough him/her. It explains that it is benevolent and needs their help in a quest and that it will stop possessing the PC when the task is done. Great if a player is gone for a number of sessions.