#105: "The Smell of Burning Heir"
It's a signed, notarized document designating that whoever bears the will as one of the heirs to one of the richest man in (insert nearby city here) who died a few days ago. The death was totally natural, old age took it's toll.
At the reading, all the heirs will be told that 1 week from then, the entire fortune will be divided between those who show up at the base of (insert nearby mountain/large hill/other large land area here).
The dead man's other heirs are all people who he hated, which seems very strange to his butler (Very high Bluff or Diplomacy check to find this out. A good butler doesn't reveal secrets). The man's home is left to the butler.
After an adventure where the other heirs either try to kill the others themselves or hire mercenaries to do so, the PCs and anyone they've made peace with will be ready to hear the final will.
It's a treasure map leading to a cave with a permanent Antimagic Fieldin it. As the party goes further down, the light fades until even Darkvision doesn't work and they have to break out torches.
It's here that the treasure lies: millions of Gold Pieces worth of Alchemist's Fire and oily rags.
Unless you want everyone to die, you should establish a way for the PCs to get out, but the place should explode.
The whole thing was one last bit of revenge.
For their trouble, any other heirs the PCs either let live after their murder attempts or rescued will pay them to leave the city so they can put the whole thing in the past.