So one of my players has a supposedly enchanted quarter staff but hasn't been able to find anything out about it except it is going to be very useful. Outside of the enchantment I'm going to get it, (like a +1 to hit a +2 to spell cast and once a day it can be used to knock all creatures within 10 feet of the target prone on a failed save) I was thinking about having it be cursed, but in the way of actually being possessed by a creature, I am just not sure what kind of creature to have it be, the party just got done with a big demon plot so that could seem redundant.

I was also kind of leaning towards a Lich perhaps? I already have an adventure planned around a cult of Orcus that is turning people into feral undead and was planning on that being headed up by a Lich, perhaps I could entwine these two?

Should there be a penalty for the player using this cursed weapon or simply over time from keeping the item on themselves? Maybe the creature makes itself known and offers great power if he can be freed? Any and all input would be greatly appreciated. 

Go with angry dryad spirit trapped in the wood.

Yeah how did you describe it?  A quarterstaff does not need to be made of wood, but if you described it as made of wood, then that changes (and potentially limits) things vs if it's made of bone, or metal, or coral from the elemental plane of water, or crystal, or stone, or something even more exotic like "tears of infinite sadness coalesced into a solid through a permanent barrier of magical force" or "a polymerized semisolid whose form in appreciable dimensions responds to your mental commands and emotions" or "a continuously swirling localized vortex whose air force at its outer barrier may as well be solid" or something like that.

Once I know how you described it I can weigh in, but if it's wood, think sylvan or fey, so angry dryad seems good, hags are always a classic, and so forth.

If it's made of wood and you have said that it is and you don't like that anymore, then burn it. The wood burns away and there's what you want underneath.

PS - I'm always a sucker for a good Cult of Orcus story.

Have the weapon be a creation from Puck, he's pretty good at creating mischief in the Feywyld. Let's say he created this weapon at the behest of Oberon and somehow it slipped through the veil. One of Oberon's lieutenants needed a new weapon, but no one should come in possession of a weapon of a court guard. Puck put a safe guard on the weapon, it would become unruly and a bother if it ends up in the wrong hands. As this weapon wasn't stolen, it is simply lost, have the weapon act like a lost, scared, and extremely annoying child. Just think:

Player: I want to slam the staff into the ground, using the charge to knock everything prone.

DM: Alright roll to hit.

Staff: OW! Don't you have any clue how much that hurts! How would you like it if I bounced you off the ground whenever I wanted...I wanna go home, you're a big meanie!

It it's made of wood then I really like the idea of having whatever creature you're going for (possibly the lich) be 'killed' and buried in forest. Then over the years a tree grew on/near the grave and the roots allowed the lich to harvest and occupy the tree. The tree was later cut down and made into the quarterstaff which your PC now holds.

If you want to dodge the whole demon aspect to avoid beating that horse to death, there's undead (like many have put forward thus far), fey (suggested by a few and my favorite go-to for demon-esque encounters when I don't want to actually use a demon; some fey are friggin terrifying, man), or an abberration (Ia!  Ia! Ph'nglui mglw'nafh Cthulhu R'lyeh wgah'nagl fhtagn).

I described the staff as "just a plain looking staff" but my players are such that if attuning to the poor warlock changed its appearance and material make up they'd just get ramped up with how cool it is. So i can go any which way. 

These are some great ideas. Even harder to decide now lol.

Then as ridiculous as this might sound (but sometimes ridiculous can be fun) why not go with everything?  The staff could be an artifact of some kind that several factions want to get their mitts on.  So every time one of them DID, they put a curse on it to attempt to avoid other factions from getting a hold of it, only to find out at some later point they already had it once and did the same.

😂🤔 that is so evil 😎

Yes, but it could also work out in your players' favor if they could manage to turn those factions after the staff against each other.

I think a good question to ask yourself csheets is:  will the staff play into the story arc currently happening or one coming down the road?  Once you have answered the question, then you can start to design not only the next arc, but the creature that is inside of it.

For example:  in a campaign I ran, there was a locket that was cursed by a jilted lover.  The curse was designed to make the attuned person feel the same anger, frustration, hatred, and sadness (in addition to never being able to be removed) as the witch who laid the curse on the locket.  It also had the effect of trapping part of the soul of the lover and when she died, and that became her avenue to seek revenge on the descendants of the one that hurt her (i.e. the locket would talk to the owner and try to guide them towards revenge).  This lead to my player having crazy mood swings, becoming aggressive whenever someone showed affection to anyone other than him, and brought the party to try to find a way remove the spirit because it was starting to try to take over the owner's body.

The arc was fun because the person wearing it was not the leader type PC and they were being forced to communicate, make decisions, and guide the group.  This also lead to a confrontation in the Spirit Realm with the Witch and an awesome spirit battle between the Witch and the group.

Hope this helps!

Thanks everyone for their help!