Ideas for a powerful spirit controlled by a more powerful demon?

[Jellyburd]

Setting up my first major plot. (new DM) I want the general premise to be that my PCs are tricked into collecting artifacts for a spirit (I was thinking nature based as my first quest involves druids) concerned that they are growing in influence and evil power and are (obviously) dangerous.
However I want this spirit to be possessed by a demon, using it to collect the required materials to summon it.
Any suggestions on creatures? or gods or religions I should check out?

I plan on them all receiving visions from this creature (that tie to their back story) as a way to bring them together. Where they will unknowingly face a trial for the creature. After the trial it would present itself and give them the quest.

[Assenzio]

Hello there! My first post!

Does your mastermind has to be a demon? If not take a read of Aboleth on MM, it makes a very good puppeteer who acts in the background. It has even a lovercraftian taste, so you can easily treat it like a supernatural entity.

[Jellyburd]

Hello there! My first post!

Does your mastermind has to be a demon? If not take a read of Aboleth on MM, it makes a very good puppeteer who acts in the background. It has even a lovercraftian taste, so you can easily treat it like a supernatural entity.

Thanks i'll check it out!
it doesn't have to be a demon. I just wanted it to get tainted or manipulated somehow into summoning the real villain. Maybe even over time as they give the spirit more of the corrupt items it eventually corrupts him/her

[Ninja_Prawn]

Hmm... other likely puppet-masters include devils of all stripes, rakshasas, vampires and green dragons (although they would probably prefer to do the manipulation in-person). Liches are known to collect magic items, but may be beyond your group's ability to fight.

[Jellyburd]

Hmm... other likely puppet-masters include devils of all stripes, rakshasas, vampires and green dragons (although they would probably prefer to do the manipulation in-person). Liches are known to collect magic items, but may be beyond your group's ability to fight.

I was thinking of having each artifact guarded by different sort of colts or beasts that relate to the item somehow. A liche could play into it later when they get stronger.
I'm trying to figure out a wise ancient creature (think Paarthurnax from skyrim, ancient wise but could still kick your ass.) not really wanting an actual dragon.

Secondly, a bigger baddy that could control/manipulate him behind the scenes.