Scientists achieved success in Automata and steam-powered robotics well before the Fair, creating artificial machines that could move and perform basic tasks. These machines were interesting and entertaining, but lacked the complexity to be truly useful.
The discovery of Ectoplasm led to an immediate leap in sophistication. As a supernatural substance, ectoplasm could lend unplanned power and abilities to the simplest machines, and in large enough quantities could create a minimal level of intelligent, artificial life. The search for more and more powerful ectoplasmic designs led scientists into the dark side of the paranormal, calling up spirits from dark and Hellish realms. An increasing number of strange infernal events began to terrify the public, but scientists couldn't let up their search for greater supernatural power. One group finally began to integrate ectoplasm directly with human brains in an attempt to create total artificial life, and their success - albeit destined for disaster - led them to present the main event at the World's Fair.
Most exhibits at the Fair involved new and improved automata and android models, featuring the most complex humanoid robots to date. Most androids at this point were integrated with large amounts of ectoplasm to provide a convincing level of artificial intelligence.
The main unveiling involved the disastrous design of accelerated human brain growth and infernal ectoplasm of great power (most likely from a Revenant Lord). The brain waited cunningly for the best moment of opportunity - its unveiling before a panel of scientists, politically-connected humans, and the public. Its dark powers allowed it to control all nearby people and automata and begin a swift campaign of destruction against the Fair. Many of the humans under its control were powerful members of society and allowed to run free, only to have a nugget of evil implanted in their subconscious that would rise up later in distant lands. So while the Fair didn't immediately cause the apocalypse, it was responsible for convincing a large number of scientists and leaders to pursue increasingly deranged research. These experiments resulted in an exponentially growing worldwide population of networked evil brains, and finally tipped the balance for their apocalyptic coup.