Avatar the last airbender

(Note: all characters are aged up for this. Events can happen au to the actually story so for example katara would be 18 whe. She frees Aang.)

The world of Avatar is filled with many spirits and odd creatures. It just so happens some of these creatures also possess the ability to possess other beings. Whether it be spirits that represent literal forces of nature such the spirits of light and darkness, Raavu and Vaatu or spirits that are merely deceased humans, the world is no stranger to them.

What sort of being decides to start a body jacking adventure?