1st Dark Whispers
The demon is nothing more than a whisper, a dark thought. Even the demon may only be passingly aware of the possession, knowing it has secured a minor foothold in the mind of the victim. If the demon is interested in this subject, they can lurk and potentially nudge the victim towards greater mental instability, to allow for the demon to progress further.
2nd Terrible Voices
The Demon has made its presence clearly known, though its nature may be unclear. At the very least it may seem like the victim is going insane. It speaks to the the victim clearly and may tease, torment and insult. It still cannot control anything, though it may be able to persuade its victim.
3rd The Battle
This is the Contested phase, where the Demon exerts sufficient influence to control the victim's behaviour. This is constantly contested, with the victim able to maintain control most of the time. When intoxicated, exausted or under heavy stress the Demon may be able to seize temporary control.
4th The Puppet
The victim’s soul is now trapped in the body, a passenger while the demon makes use of the host. It may intentionally relax its control to torment the victim or accomplish other goals.
5th Body and Soul
This degree of possession is generally fatal, as the Demon has fully taken the host’s body and converted it fully to their own, granting it a material body in our realm. Exorcism as we know it is no longer possible, unless the beings doing so possess magic powerful enough to push out the demonic spirit and reform the victim’s body. This is the power of gods. Thankfully, it is a rare demon that is capable of this form of possession.
5th Degree possession is usually short lived. The hosts usually are not capable of holding beings of this power and so their forms will degrade - eventually causing the demon to ‘die’ and return to the realms.
Each Degree - apart from 5th degrees - can be broken up into sub stages to track the progress of the possession. Most demons have a threshold they can reach, though circumstances - locations, insanity, rituals, magic items - could boost or reduce their power.