|“||The title of [Sole God] bestowed upon the winner of the war, every God's desire, the Star Cup.||”|
The Star Cup or otherwise known as the Suniaster (スーニーアスター, sūnīasutā) is an ancient relic that according to legend, when obtained will grant the user the title of One True God and absolute power above all other races.
The quest for the Star Cup is believed by many to be the main reason why the Old Deus mobilized armies and plunged the world into The Great War. Though its existence has been speculated upon for many eons, Riku is able to figure out that the Star Cup is actually a term for the core of the planet's Spirit Circuit network which provides all living beings on Disboard with magic. Thus, he concludes that if one is able to possess the core, they will obtain more than enough power to call themselves the One True God.
To that end, Riku spends a year discreetly planning and manipulating the movements of the other races' armies. His goal was to force the armies into a situation where they all had to unleash their ultimate attacks on each other, which would then be directed by the Ex-Machina towards the planet's physical core, therefore damaging the core of the Spirit Circuit network and forcing the Star Cup to reveal itself. However, this plan was eventually found to be flawed (not in the plan itself, but the fact that enough of the devices for redirecting the blast had not [and due to the increased presence of the races, could not be] been placed, thus they could not redirect the combined attacks at the planet), so instead of attempting to control the combined wave of ultimate attacks, the Ex-Machina reproduced a version of the combination attack in the form of a massive gun called the Starslayer, which they entrusted to Riku.
Using the Starslayer, Riku penetrated the planet's core and released the reserves of magic energy stored within the planet, which were compressed and gathered into a polygonal crystal RIku believed to be the Star Cup. Deeming himself unworthy of the title of One True God, Riku's strong wish for the existence of an 'ultimate gamer' and the incredible surge of magic energy pouring out from the planet's core gave birth to the final and weakest Old Deus - Tet. Promising to play a game with Riku and Shuvi in the future with a smile, Tet claimed ownership of the Star Cup and used its power to lay down the Ten Pledges, thus ending The Great War and fulfilling Riku and Shuvi's dream of a peaceful world.
According to the movie No Game No Life: Zero, the Suniaster is a Conceptual Device, which gives complete power and authority to its holder. It was made so that it would only physically appear before the most powerful existence. The Deus thus believe that the only way they can obtain it is by killing all other Deus, thus the survivor would be the most powerful existence. Riku's plan was to create a hole into the planet revealing the World's Spirit Circuit, since all the Deus were created by it.