The planetary alignment is a celestial event that occurs periodically within the solar system where the six outer planets beyond Earth align. The event was prophesied by the Fates to occur again 18 years after Hercules' birth and would allow Hades to conduct a "hostile takeover" against Mt. Olympus with the help of the Titans.
Energy released by the planets lining up created a vortex that moved the water away from the place in the ocean where the Titans had been imprisoned by Zeus back when the Earth was new. Hades then broke the energy gate covering their prison and the Titans climbed free to begin an assault on the home of the gods. However, in their anger, they initially headed in the wrong direction until Hades got them turned around.