HH Holmes became known as the Devil in the White City because of the legend that he built a murder castle in Chicago in 1893 with the intent of murdering hundreds of people as they visited the Chicago World Columbian Exposition (which became known as the White City).
He built a hotel with secret walls and rooms so that he could kill people and dispose of their bodies in the basement.
At least, this is the story we've all been told. The real story is quite different.