The Vortex Offensive refers to the attempted take-over of Earth by the Vortex immediately following the events of Ecco the Dolphin. It is the over-arching plotline to the sequel Ecco: The Tides of Time.
Following Ecco's defeat of the Vortex Queen in Ecco the Dolphin, the Queen follows Ecco and his pod back to Earth. Once there, she begins to take root upon the Earth for Vortexkind. The Vortex Queen, aware of the Asterite's contribution to her previous defeat, attacks and destroys the Asterite. In addition, the Queen spawns and creates a new Hive, and the Vortex begin to feed upon life on Earth. During Ecco's travels, several different stories of attacks by "strange creatures," or even direct references to "Vortex creatures" can be heard.
After fully restoring the Asterite, Ecco's powers are restored and the Asterite sings a powerful song to call all singers to arms. This became the Cetacean Counter-attack. With other singers occupying a majority of the Vortex, Ecco infiltrates the new Hive and attacks the Vortex Queen. Despite the destruction of the new hive, and unbeknowest to Ecco, the Queen survives in the form of a Vortex larvae and travels to the Atlantean Time Machine where she escapes into the past.