Yakusoku is a terrestrial world operating as the capital of The State of Tenshi, a fairly small state within the Confederacy of Borealis. The planet has a small population of 1.2 billion, however, these 1.2 billion people together have one of the highest concentrations of wealth on a single planet in the known universe, with an estimated 25 Octillion C-Units divided among these individuals. The planet is located within Kanadett, a star cluster in the Via Sagittaria Galaxy.
The planet is entirely hollowed out, with a framework of active support holding the ten-kilometer-deep crust up over the core. The core itself was replaced with several layers of radiation shielding and a black hole. The black hole itself has a small accretion disk which acts as a power source for the planet, its potential output rivaling that of its parent star. The black hole was originally constructed in case of a siege cutting off the planet from its star. In the event of an emergency, the black hole is capable of emitting large amounts of energy in the form of a beam of energy. Most of the defense infrastructure on the planet was built during a more turbulent time of the galaxy. Many recently colonized planets do not have such extensive (and more importantly expensive to maintain) defensive structures.
Even with the defense structures, the large concentration of wealth on the planet warrants extra precautions. The Nobility residing on the planet has vast numbers of ships from the State stored in the system, capable of responding to a threat in minutes. They are designed to defend the planet long enough for the most important individuals to escape while allowing for a large fleet to eventually return to bolster the planet's already extensive defenses. A large portion of the energy of its star, Senta, is devoted to a high-energy laser array capable of vaporizing any covalent bond from over an astronomical unit away. Senta is a very luminous star, allowing for a large number of energy-intensive infrastructure to be built around it. This Beam, a variation of a Nicoll-Dyson beam, is one of the most powerful weapons of its type in the Via Sagittaria.
The planet itself is covered in large nature preserves with flora and fauna. While virtual reality is enough for the average person to experience nature, those that can often appreciate "natural" beauty over the artificial.
Yakusoku has three major continents, Datte, Gerin, and Nepputsk accompanied by hundreds of smaller islands. None of these continents were present when the planet first formed as terraforming typically disrupts planets so much that even the largest natural structures are often changed beyond recognition. What little magnetic field the planet had was destroyed by the hollowing out of the planet, so it is protected from damaging rays by a large array of magnetic field generators orbiting it.
The oceans are filled with artificially enhanced coral designed to live in not just shallow water but across most of the ocean floor, creating stunning reefs near the surface and equally stunning bioluminescent reefs towards the bottom. Large kelp forests fill more temperate areas of the sea floor.
The planet was first colonized in 38 BCE by the H. Aimer Corporation to act as a residential world for its most important stock holders and their families. The Via Sagittaria Galaxy was in the midst of the deadly War of the Final Transition, causing anyone wealthy enough to flee the war to flee.
When the Yohjan Confederacy collapsed in 0 CE, so too did H. Aimer Corporation, causing the entire star cluster to shatter into a large number of competing organizations. These organizations soon reorganized themselves into monarchies. On Yakusoku, four families organized themselves into noble organizations, the Takanashi Family, the Serebryakov Family, the Sakagami Family, and the Claes Family. These noble families were generally amicable to one another given the relatively small territory each one controlled. As the rest of the cluster fell into a nonstop series of wars, Yakusoku remained one of the few orderly areas of the cluster. In 2,304 CE, the four rulers of each family met together to formally unite their kingdoms into one state. They united themselves into the Tenshin Ministry. This ministry would go on to form the State of Tenshi.
Yakusoku soon became the capital of an expansive interstellar empire. The State of Tenshi united Kanadett in 4,012 CE, cementing Yakusoku as the political center of Kanadett. Using infrastructure left over from H. Aimer Corporation as well as weapons left over from the Triumvirate Civilization, Tenshi went on to become a great power capable of influence on a galactic scale, only earning Yakusoku an even more prestigious spot on the galactic stage. As Tenshi expanded, the infrastructure on Yakusoku, both defensive and civil, became some of the most advanced infrastructure in the galaxy. Other capital worlds within Tenshi, such as Ayomi and Hikari all had wormholes leading to Yakusoku.
The planet received minor damage from both the Great Empyreal Crusade and the War of the Ancients