I've been formulating a campaign together for a while and I currently like where it's headed. My Players are going to start it at the end of this week. Then I got a really cool idea that I would like to attempt to add (like a sort of spice in the story).

To give a very brief and simple summary of one of my NPCs that the Players will encourage. This NPC was kidnapped at a young age by a cult. The cult attempted to summon their 'Great One', but failed. Instead, another spirit entered the NPCs body (this spirit is friendly). The NPC can see and hear the spirit, and the same can be said in the opposite direction. However, no one else can hear or see the spirit (looking into the Ethereal Plane also won't work because she is no longer there). Furthermore, the NPC and the spirit can swap places whenever they like. (Ex. Currently, NPC is in control of their body and the Spirit floats around and such. To compare it to something, this can be viewed as similar to an out-of-body experience. When both agree, they swap places. The spirit takes control of the NPCs body and the NPC becomes a spirit. In other words, the roles are reversed.) The spirit was also once human before and she still has her memories.

The NPC could not use magic before, however, the spirit could use 'Dark Energy Magic' (a special kind of magic). The NPC uses her newfound abilities, from the spirit, to escape the cult's grasp. I have it where the NPC's body can use this magic similar to any other magic. However, should I have something unique for the other (the one in spirit form)? Right now, I have it where the Spirit can travel a set distance away from the NPC (FYI, if the NPC dies, the spirit dies too. Their souls are connected). The spirit can feel the same pain as the NPC (Ex. pain in the arm), and they can sense what the other is feeling. Moreover, the spirit cannot interact with objects, as they would phase right through them. However, if they concentrate, they can only push/pull nonliving organisms up to 5 lbs. 

What else do you think I should do with this, or should this be it?

I wouldn't put too much main plot stuff into one npc. It can make the game too much about the dm's creation rather than about the player character's story. 

I think you have a cool concept for this npc, I would just try to consider how to fit them more in a back end support role for the party. Maybe they have good exposition about the main plot they can give the players once they've earned her trust, and then the players can do with that information/trust what they will. 

I wouldn't try to cement this npc's role too hard because then the story can become too rigid, and if the players behave unpredictably then things can go south quick. 

That makes sense, I kind of figured. My plan was for this NPC to be used for a side story. I was thinking of structuring my campaign like this. Having the main plot still be there, however, if the Players feel interested in certain topics/events/NPCs, there could be a side-quest that they can do that follows or gives information (like backstory or something) that follows. (Also, doing so would affect the main quest too). Thanks for the advice.

But ability-wise, should whoever is in 'spirit-form' have anything other than just 'slightly push/pull objects'?

Maybe they would have special knowledge, or way of attaining knowledge by way of the spirit world or something. Maybe they can see glimmers into people's pasts in a way that can potentially help the party. 

Quote from OverHandFungus >>

 

What else do you think I should do with this, or should this be it?

I'd treat it as the typical "fractured memory" trope.

So they encounter this NPC somewhere (she? he? they?) have been captured by Monster_XYZ, when the party comes along and saves them.

They basically explain that they're not sure what happened - but they can't remember much of who they are. They would appreciate the player character's help in restoring their memory.

She has a vague memory of a location.

This location leads them back to the cult area. Where the person was initially to be sacrificed.

Party fights off some crazy cultists.

The NPC gains a piece of their memory.

Recalls some old chapel.

Party heads that way, fights things along the way. The whole time, the NPC is doing minimal to help - but all magic related.

And reveals during a rest, that they don't recall being able to do magic before.

Get to said old chapel, and the NPC gets another memory back.

Rinse and repeat, as  they gain memory pieces, they become a little more helpful (as the party is leveling).

They would just be an NPC to ride along with the party, and be able to feed the party information (as information from the Etherplane is pouring into their brain). 

Like, "Hey, there's someone in danger in the mountains to the west. We should go help."

And they prove to be right - which should make the party wonder how this NPC knew this.

Stuff like that.

But definitely make the NPC not powerful and not super helpful during combat.

The star of your game isn't your NPC - it's the players around the table with their characters.

Thanks for the info, I really like the ideas. I'm going to find a way to combine the two ideas and see what works. Thanks again

Actually, how could that idea work, SirTawmis (it's not a bad idea. I really like it in fact)? If the NPC lost her memory, couldn't the spirit-like girl following her explain what she know about the NPC? Maybe she forgot it because of trauma; the NPC and the spirit were on their own quest to figure out their past. NPC gets into trouble (from Monster_XYZ) and Party can save them. Then it could play out similarly to what you mentioned before.

But, how would having this 'extra soul' (two souls, one body) affect that scenario? Or am I overthinking it and it would technically not change much?

Quote from OverHandFungus >>

Actually, how could that idea work, SirTawmis (it's not a bad idea. I really like it in fact)? If the NPC lost her memory, couldn't the spirit-like girl following her explain what she know about the NPC? Maybe she forgot it because of trauma; the NPC and the spirit were on their own quest to figure out their past. NPC gets into trouble (from Monster_XYZ) and Party can save them. Then it could play out similarly to what you mentioned before.
But, how would having this 'extra soul' (two souls, one body) affect that scenario? Or am I overthinking it and it would technically not change much?

In a world of magic, you might be overthinking the two souls.

The fact that a single body is sharing two souls could be the cause for the memory fracture. Both souls is in the body - causing both the person who was almost sacrifice and the soul of the spirit sharing the body - causing the memory to fracture as to "who the real person" is. 

I see, that could work. Thanks for the help