Danny Darrow was a crazed hermit, and the surviving youngest member of the Darrow family.
He and his family were fighting over a puzzle-piece artifact that might be a clue to the haunted treasure of the conquistadors, at the time he got his hands on it, an earthquake caused their mansion to be swallowed up below ground.
Scooby-Doo! Mystery Incorporated
He somehow survived there for about 75 years, outliving his family because he was the youngest. When he was disturbed by the old Mystery Incorporated he thought they were after his treasure, and he tried to fill the mansion with traps for them, but they left while he was still working on the traps.
When the gang discovered a way down to the mansion, his crazed mind mistook them for the original Mystery Inc., and he tried to trap and kill them. They managed to escape, and eventually he realized they were not who he thought. He gave them the puzzle piece and let them escape, just as the ground around the mansion began collapsing again, leaving his fate ambiguous.