Equlilbrirum is the final mission in the main campaign of the game.
"All your work has been towards this very moment, Underlord! The Empire is in shambles; its army is decimated, its people are paralyzed with fear, and Emperor Lucius has sealed himself away in the hopes that the dark may hide him. Begin your siege of the Citadel, Underlord, and seize our final victory!"
Win your War for the Overworld.