Since 1974, diabolical creatures have evolved, gaining and losing many powers and spells through the centuries. Through all this, the mechanics of possession they are illusory
Note that all editions of D&D have had careful descriptions about possession When necessary, consider the (in) famous ghost. Explain how the living target creature would lose control, what statistics change (or not) and how the ethereal form is transformed into a material body. The mechanics are demanding and have a really effective game usage. That's great.
Contrast this with the demon 5e Shadow: in the writing they claim to have some ability to possess goals, however, it seems that mechanisms for this ability are lacking. Most other demons seem to be devoid of all mind control powers all together. Even the only mental alteration of the almighty Pit Fiend does not extend beyond causing enemies to have a little fear … for six possible seconds. In a failed bran. Once a day. As long as one is not immune otherwise.
It would add a lot to a campaign to have someone, characters, player characters or even the players themselves (open for anything here), possessed by a demon. Keep in mind that StackExchange even presents a brilliant review on how to get rid of a demon once one has been taken over: this is a step by step / & # 39; How-To & # 39 ;. Is very good.
Long story short: How do demons possess (players or not) characters? Where are mechanics listed?