The Death Nova is a space station built by Dr. Robotnik and the Robotnik Empire. Introduced in the Blue Blur saga, it's a super weapon with the power to annihilate the entire surface of a planet, thanks to the Cannon Core. It's the primary weapon of Robotnik and an important plot-point in Blue Blur: Curse of Mobius.
Development of the Death Nova began immediately after Dr. Robotnik invaded Mobius and founded the Robotnik Empire. The intention of the Death Nova is to develop a weapon so powerful it would bring all residents of the Multiverses to their knees. It took more than five years to finish it, and the only thing left to do is to collect all seven Chaos Emeralds to make it fully operational.
The Cannon Core
The Cannon Core is the main part of the Death Nova. It holds a laser with the power to turn the entire surface of a planet to ashes. With that much power, it needs a powerful source of energy to stabilize it. Without that energy, the Cannon Core's power would be too much for the Death Nova to handle on its own, thus causing the space station to destroy itself. That's why Robotnik is going after the Chaos Emeralds.
The Death Nova Robot
The Death Nova Robot is the most powerful controllable robot Robotnik has ever built. If Sonic tries to break into the Death Nova, Robotnik would control that robot to annihilate his arch nemesis.