Species 8472 Theory
Perhaps the people at Volition watch Star Trek: Voyager. I don't know. But the Shivans do bear many similarities to species 8472, so here's my theory.
The Ancients were powerful. They conquered and destroyed, adapted and evolved. I believe that they developed subspace technology to the point of being able to use subspace to enter an alternate dimension, possibly similar to space. Ships, both Ancient and Shivan went in and out of fluidic space and normal space for as long as a portal was open. But the Shivans, with their advanced technology and physiology, were able to stop the Ancients. For the first time, the Ancients encountered resistance. Resistance was not futile.
With the Ancients gone, and the technology to enter "fluidic space" gone, many Shivans were trapped in normal space. Many, many Shivans. The Shivans adapted to normal space and developed their technology even more. Their shielding, their beam weapons, everything was honed over thousands of years. In those thousands of years, they had to strip planets of specific resources to create the technology used to hopefully return to their realm. The Sathanas juggernauts were designed to cause a star to go supernova and open a singularity to return to fluidic space. Their first attempt with the technology, about 200-500 years after the destruction of the Ancients, was a disaster. The star they caused to go supernova, which is now the wikipedia:Crab_Nebula, was not the right kind and did not create the singularity necessary to return to their realm. The infant Shivan Empire dwindled to a few ships.
But it was enough. The Shivans mapped their way across the galaxy, slowly building ships and building an empire. Over the next eight and a half thousand years, they built a new fleet of Sathanas-class juggernauts, to put their plan into action.
The Shivans are not stupid. No, they are quite intelligent. Their subspace communication nodes that they left behind were able to transmit sensor data clear across the galaxy. When they realized the Terrans and Vasudans were going to develop space-faring technology, they predicted that they would also discover subspace nodes and explore the galaxy. Not wanting to needlessly destroy the Terrans and Vasudans, they constructed a new ship on their way back.
The new ship, the Lucifer, with its shielding that is impervious to plasma, electromagnetic, and projectile based weaponry, had a purpose other than mindless destruction. The supernova of Capella, the star they determined would be perfect for the job, would end life in the galaxy as we knew it, if the shockwaves in subspace were allowed to travel along the subspace conduits that connect stars. To ensure that the Terrans and Vasudans destroy the node linking Capella to the rest of the Terran-Vasudan area of the galaxy, the Shivans had to destroy a planet. Only then would the scientists-made-refugees be led to Altair, where remains of the Ancients would lead them to better subspace technology.
Only if the Terrans and Vasudans understood subspace to some level, and under threat of destruction, would they realize the importance of cutting-off the Capella system.
So Capella was cut off and went supernova, and the Shivans made it back to their own realm, hopefully never to be seen again.