The Tro-Clon was a "confluence of events" which would bring about the purification or ruination (or both) of mankind. Its arrival was announced in the Nyazian Scrolls.
It was initially assumed by the Angel Investigations staff that the Tro-Clon is was single event or an individual, though they soon learned that, in fact, the Tro-Clon was a confluence of events culminated with the rise of Jasmine, as explained by Skip. According to him, there was a series of events going back years that had all been guided by Jasmine, working as a Higher Power, towards the single purpose of bringing herself to Earth.
Fred and Lorne switching dimensions (Lorne's spell was necessary to "activate" Jasmine in the amnesiac Cordelia).