Calvert Mansion

The Calvert Mansion is a location in Point Lookout which will be added to Fallout 3 in the Point Lookout add-on.

It is inhabited by a single ghoul called Desmond and who has been attacked numerous times by a group of tribals, whose aim is to burn the mansion to the ground  According to Tobar, the building holds hidden treasure.

The Mansion is one of the largest on the island, the other being the Blackheart Mansion.

Spoilers follow. You have been warned.

Roughly near the end of the The Local Flavor/A Meeting of the Minds quest, after investigating the leader of the tribals, you will return to the masnion only to have it blow up in your face. It seems that Professor Calvert made another attack against Desmond forcing him to retreat to his panic room, and activate the failsafe which destroys the mansion. 200 years of scheming, plus the death of his two dogs, causes Desmond to be filled with a terrible resolve. He'll tell you that he has found Calvert's secret base, which is located under the Point Lookout Lighthouse, in order to settle with him. At the end of the turret and robot filled tunnels you will find the Professor's brain, suspended in a large tank. It is a standoff, and only you can break it. Choose to side with Desmond, and he will give you the key to the locker beyond the brain chamber. Choose Calvert's side, and he will try to kill you anyway.

Appearances
The Calvert Mansion appears in the Fallout 3 add-on Point Lookout.