The reason I'm asking is because of the Haunted One background. One of the options on the Harrowing Event table states:
A fiend possessed you as a child. You were locked away but escaped. The fiend is still inside you, but now you try to keep it locked away.
Is it possible for an incubus or another fiend to possess a mortal being?
The Haunted one background indeed gives you access to being possessed by a fiend, but the nature of that fiend is entirely unclear. No published monster of any sort can possess a creature as described in the background text presented, let alone a fiend. Beyond being able to properly possess a mortal (e.g. as Magic Jar or an intellect devourer might), such a being also needs to be able to be fought off by the possessed person after possessing them, yet thereafter remain with the possessed one.
There is, however, one sort of being that can do all these things: sentient magic items. Sentient magic items in 5e are maybe creatures and maybe not, but they are definitely NPCs and universally capable of having something called a 'personality conflict' with their wielder and also capable of being attached to characters beyond simple proximity (see cursed items in the DMG).
Regarding personality conflicts:
A sentient item has a will of its own, shaped by its personality and alignment. If its wielder acts in a manner opposed to the item’s alignment or purpose, conflict can arise. When such a conflict occurs, the item makes a Charisma check contested by the wielder’s Charisma check. If the item wins the contest, it makes one or more of the following demands:
- The item insists on being carried or worn at all times.
- The item demands that its wielder dispose of anything the item finds repugnant.
- The item demands that its wielder pursue the item’s goals to the exclusion of all other goals.
- The item demands to be given to someone else.
If its wielder refuses to comply with the item’s wishes, the item can do any or all of the following:
- Make it impossible for its wielder to attune to it.
- Suppress one or more of its activated properties.
- Attempt to take control of its wielder.
If a sentient item attempts to take control of its wielder, the wielder must make a Charisma Saving Throw, with a DC equal to 12 + the item’s Charisma modifier. On a failed save, the wielder is Charmed by the item for 1d12 hours. While Charmed, the wielder must try to follow the item’s commands. If the wielder takes damage, it can repeat the Saving Throw, ending the effect on a success. Whether the attempt to control its user succeeds or fails, the item can’t use this power again until the next dawn.
This system models the exact sort of possession described in the Haunted background, and is the only such system in the game. Regardless of what sort of being you decide is possessing your player's Haunted One, I strongly recommend you use this system as a baseline to model the possession.
First the easy part: A succubus/incubus has the ability to charm a creature for a period of 24 hours, and the target under this effect does whatever the succubus/incubus says, up to and including suicide (although it does get a saving throw before performing a deliberately harmful action.)
Source: MM pg. 285
Charm. One humanoid the fiend can see within 30 feet of it must succeed on a DC 15 Wisdom saving throw or be magically charmed for 1 day. The charmed target obeys the fiend's verbal or telepathic commands. If the target suffers any harm or receives a suicidal command, it can repeat the saving throw, ending the effect on a success. If the target successfully saves against the effect, or if the effect on it ends, the target is immune to this fiend's Charm for the next 24 hours. The fiend can have only one target charmed at a time. If it charms another, the effect on the previous target ends.
Second: They are of the creature type Fiend so they meet that requirement.
As for the preference I mentioned above, that actually falls under the flavour of how a succubus/incubus acts in some realms. There's a rather involved write up which details how the succubus/incubus goes about seducing a victim, and the focus on how the succubus/incubus wants the victim to succumb willingly. The goal of a succubus/incubus is to get the victim to give in to temptations that they otherwise wouldn't. Because of this succubi and incubi like to target virtuous individuals as their souls are the sweetest once corrupted.
Pertinent text:
The succubus or incubus resorts to charming a victim magically only when necessary, usually as a form of self-defense. A charmed creature isn't responsible for its actions, so forcing it to behave against its will won't bring the fiend closer to the ultimate prize: the victim's soul.
So as for your question: Yes, it's entirely possible for you to have charmed a target that then became wholly infatuated or dedicated to you. Nothing about the charm effect indicates you know it was magical or that you know the succubus/incubus did it to you, so if you are playing a character who is "possessed" by a succubus/incubus it could simply be a character who has been targetted and is routinely visited by said creature.
How would the succubus/incubus accomplish this task daily? Etherealness or shapechange, it possesses both abilities.
Etherealness. The fiend magically enters the Ethereal Plane from the Material Plane, or vice versa.
Shapechanger. The fiend can use its action to polymorph into a Small or Medium humanoid, or back into its true form. Without wings, the fiend loses its flying speed. Other than its size and speed, its statistics are the same in each form. Any equipment it is wearing or carrying isn't transformed. It reverts to its true form if it dies.
It can either visit its target when it's alone using etherealness to revert back to the material plane in order to charm it's target, or it could simply shapechange into a small or medium humanoid, like a damsel in distress or a small child looking for it's mother and fleeing monsters. Either way, it can actively charm it's target without giving itself away effortlessly.
Succubus and Incubus are one of my favourite monsters in the game because of how devious and devastating they can be when wielded right. I hope you enjoy playing your character. Having a succubus/incubi follow you about acting as a possessing element is very interesting and creative.
