The Pirates' Den (formally House of Pirates) is a location in the Overworld of Dragon Quest III. It is home to a band of pirates who are only present at night. The house sits on the South American continent near what would be Argentina in the real world. The House of Pirates is the location of the Red Orb.
The Red Orb is hidden in a secret basement outside the house. It can be found by searching the ground after pushing a boulder out of the way. A pirate in the house mentions a red orb, but can't remember where he saw it. It is also common to use the Echoing Flute to learn of the orb's location at the Pirates' Den.