For this one, go to the shutter door on the Happy Mine and interact with some nearby barrels, the order is:
- Bottom Left
- Middle Left
- Top Left
- Middle Right
Once you've done that, the Keyman will reveal himself and ask for 1000 Bandit Bucks, which is the money you see all throughout the game, once you collect 1000 Bandit Bucks, go back to the Keyman, he'll open the door and you'll find a golden block, once collected, you'll get a skeletal outfit called Dead Servant and the achievement.