This adventure was written and played in Eberron originally. I modified this so it can be placed anywhere, though this adventure has some epic fail consequences if the players are idiots... So beware.
The players are having a good time at a tavern when a voice speaks to them from the nearby table. The table is empty, except for one man who lies slumped over the table with a shield under him.
The voice, it explains, belongs to the shield (Chaotic Neutral +3 shield able to cast spark) which is SUPER INTELLIGENT (or so it claims). It presents itself as Maxirallion the Legendary. It claims to have protected a former owner against the flames of a black dragon (hint: black dragon's don't have flames). It says it is tired of its current owner and wishes to seek out adventure and wealth. It points them in the direction of a particular gambling den in a city named <enter name here> owned by a smuggler named Warzu. Beneath the gambling den lies Warzu's smuggler hideout and beneath that lies a system of tunnels and caves.
Maxirallion tells them that he and one of his former owners hid something of immense value down there and if the players do as he says they will be richly rewarded.
The PCs infiltrate the gambling den, sneak past some of the guards inside and find the secret compound below. They proceed, fight some monsters in the caves and find some loot... until finally...
They reach a large cavern with a large column in the middle around which has been piled kegs upon kegs of black powder and a long fuse... Maxirallion urges them to look closer, because his treasure was hidden near the column. Maxirallion says something like this:
"Finally! After almost ten years of hard work and toil the ultimate plan has come to fruition!"
The players are likely to be quite confused at this point. In either case one of them might have him equipped, otherwise he asks to see it himself. If he's within reach of the fuse, he casts spark. The fuse will take about 10 seconds until the gunpowder detonates, blasting the pillar holding the upper levels stable, collapsing the gambling den, the smuggler hideout below, and likely a considerable amount of other buildings along with it.
Any alert player should be able to stop the fuse unless they all decide to run. If the fuse is stopped the shield is like "Nooooo! My ultmate plan is ruined!"
Then he reveals that as a shield he changed owner many times for many reasons (mainly his previous owner getting horribly murdered or killed by monsters) until he is finally... And at this point Maxirallion takes a long pause... "Until I was finally sold for a pittance by Warzu! Such an insult... It is unacceptable!"
Ever since that he's spent the last ten years plotting his revenge, manipulating his previous owners with lies to get them to do what he wanted and slowly undermining the tavern until one of his last masters were mortally wounded and died as soon as he got out, forcing Maxirallion to start working on another master.
There's some loot down here hidden by Maxirallion's previous masters, and they can sell the shield for money, or keep it (it's +3 after all).
This adventure was originally written to take place in Sharn, so Warzu's den of gambling is placed in the middle levels of a large tower. Hence the conquences for the players failing to stop the burning fuse is the destruction of an entire tower and the potential death of hundreds of people. The adventure there does also not take place underground, which changes things considerably...