One member of the party (It is the GM's option who it might be or it could be randomized. Most likely, it will be one who is very interested in money and who spends time in bars and/or gambling amid the dregs of the city) meets an elderly dwarf named Durin Dryden in a card game. They find over time that get along well and the old fellow starts a half-drunken monologue when they are alone very late in a tavern in Iron Town Under.
"Oh, lad, it is my shame that I never followed my dreams beyond the next pint of ale and the next deal of the cards. I had in my hands, IN MY HANDS, a key to great wealth but I got distracted and have wasted the last two centuries lollygagging around Iron Town when I could have, should have, taken a chance and won a fortune."
Nothing comes of that immediately, even if the player-character tries to find out more, but two weeks later a dwarf mutual acquaintance tells the player-character "Old Durin thinks he's breathin' his last, and it seems like he might be right. He has things he wants to tell ya."
If the player-character hurries to the old dwarf's side, he finds him stone sober and breathing and speaking with difficulty.
"When the great Lords of Glon' grew tired of the raiding and looting by the Baron of Mondragon, they sent an army against the Baron and defeated him, even though he had Svein Firehair and his sworn men fighting for him. It was a fierce battle, I'll tell you, for I was young and foolish and marched with the District Militia. Many brave men and dwarfs and many who may not have been sa brave fell that day and the commanders could barely restrain the men, and even the dwarfs, from slaughtering prisoners because the fighting had been sa fierce.
Me, I was for mercy and for following the orders but men get more excited than we do, that's why we are called the moderate people. So, I had to keep a couple of the fellows from finishing off a wounded lad and making sport with his sister. The boy thanked me but he was clearly going to die before any healer could get to him, that's the sorrow and the pity and him no older than forty. He gave me this"
And the old dwarf handed the character a brass key.
"And told me that he had served the Baron as his treasurer after his father, who had held the job, had died. He said that there was a room with great treasure down under the Keep and that he wanted me to get the treasure. One thing, the treasure room door is not visible. It is in the middle of the right-hand wall of the main room of that floor and the key will fit in what looks like a crack in the plaster.
He might a' told me what level it was on but I've forgotten. Damn my old mind.
I could not be getting any great treasure with that army all around the place, not and keep it, so I was going to have to wait and come back. Besides, he charged me with getting his sister to relatives here in the District and I did that. And I never got back to get the treasure. I want ya to have it, lad, but you'd better gather friends for the way to that keep is rarely traveled the now and there are dangers. And who knows what is in a keep that is deserted for two centuries?"
Then, the old dwarf wanted to play cards and they pegged a board for a few hours until the old fellow's breathing got even worse and he took a drink and lay back "to rest a moment" and died.
https://www.drivethrurpg.com/product/312156/Mondragon-Keep?manufacturers_id=11833