Master of Orion II: Battle at Antares

Story

Long before the time in which the game is set, two extremely powerful races, the Orions and the Antarans, waged a war that devastated much of the galaxy. The victorious Orions didn't exterminate the Antarans, but imprisoned them in a pocket dimension before leaving the galaxy, leaving behind a very powerful robotic warship, the Guardian, to protect their homeworld. Some time after the game starts, the Antarans, having broken out of their prison dimension, start sending increasingly powerful fleets against the players' colonies to destroy them instead of invading. The only way to stop them is to take the fight to their home universe through a dimensional portal.