What RPG would fit this one-shot idea?

[hopeless]

Wraith the Oblivion I think it was called a White Wolf Publishing game using the same system as Vampire the Masquerade and the others they do.
That was my first reaction to the opening premise.

 

[Fenris-77]

I've heard really good things about Ghosts of Albion, and I own it, but I've only breezed through it. It's a Cinematic Unisystem, and it has a bunch of cool widgets for running ghost PCs. The PDf is on sale on DTRPG for 10 bucks too, so there's that.

 

[aramis erak]

I've heard really good things about Ghosts of Albion, and I own it, but I've only breezed through it. It's a Cinematic Unisystem, and it has a bunch of cool widgets for running ghost PCs. The PDf is on sale on DTRPG for 10 bucks too, so there's that.

It lacks the needed bits for doing Ghosts as PCs; Angel is 99.9% compatible, but covers PC "monsters."

 

[GMMichael]

That sounds like an RPG to me.

But whatever you want to call it, I don't have the head for writing the mechanics myself. I don't know what works, what's balanced, what numbers to use. So I'm open to anything that does the above.

It's from an RPG, but you can use the four rules I mentioned (or three and a suggestion) without a specific RPG, and without writing any "mechanics." It would work like this:

Three PCs have three goals: possess a person to write a message, scare someone away, and lead the living to the secret room (where the final problem lies) - all determined by GM or PC. You would also want some landmarks to subdivide the goals (for example, scaring someone away requires three successes, because that NPC is able to regain his courage twice). When a PC achieves a landmark, like scaring someone's pants off (figuratively), the PC rolls a d8 and adds it to the total amount achieved by all PCs (the progress pool). If 3 PCs need 3 landmark successes each, you could set the maximum progress (the goal amount) at 36 (3 x 3 x half a d8), and let PCs see the total as they progress. (I would prefer, for tracking progress, to stack little monster dolls until they are able to " see" out the top of a bowl, but to each his own). After 36 progress, the door (revealed by the secret room goal) opens, and PCs get to use all their newfound ghost powers to fight the demon behind it all!