The Supreme Kai's Planet (Sacred World of the Kai (界王神界 'Kaiōshinkai', lit. "World King God World"), called Kaioshin's faraway planet in the Viz manga, is the name of the spheric world where the Supreme Kais reside) is a planet in Other World where the Supreme Kais reside. You may train here.
The Sacred World of the Kai is where the four Supreme Kais and the Grand Supreme Kai used to live, and possibly where every single Supreme Kai has lived. Most Supreme Kais do not live here anymore, as the Western Supreme Kai and the Northern Supreme Kai are both killed by Kid Buu, whilst the Southern Supreme Kai and the Grand Supreme Kai were absorbed by Kid Buu, during Bibidi's quest to rule the galaxy.
During the time the Dragon Ball series is set, the Sacred World of the Kai is home to Shin and his bodyguard Kibito, and later to Old Kai and the permanently fused form of Shin and Kibito, Kibito Kai (until he is split back into the two fusees). The two Space Lemurs that Goku freed from the Sugoroku Space, Sugoro and Shusugoro, also live on the planet of the Kais during and after the Baby Saga.
The planet has several moons and flourishes with life, with plenty of fish in its streams and rivers, as shown in Shin's flashback. It also has rocky and grassy areas, as well as icecaps, which was shown in the flashback. The planet of the Kai is incredibly sacred, literally to the point where, except in dire situations, such as the release of Majin Buu, even Grand Kai is not allowed to set foot there except deities and attendants of equal and higher rank. The planet seems to be a very empty place for a large planet, as not many reside on it. This may be because of the planet's sacred value, or because it is very difficult to get there, as it took Goku a while to find Gohan in Other World when he was looking to see if he was dead or not.
Approximately 75 Million Years prior to the events of Dragon Ball Z this planet was inhabited by Old Kai who lived there peacefully until one day an Old Witchappeared and took of one of his Potara Earrings accidentally, merging the two together permanently. Another day the God of Destruction Beerus dropped by for an administrative meeting being held once every 1,000 years between the Gods of Destruction and the Supreme Kais, and the two of them got into an argument over some trivial matter causing Beerus to get mad and seal him up inside the Z Sword.
Approximately five million years before the events of Dragon Ball Z, the Sacred World of the Kai was inhabited by all the Supreme Kai of the universe: Grand Supreme Kai, Northern Supreme Kai, Shin the Eastern Supreme Kai, Western Supreme Kai and Southern Supreme Kai. According to Shin (who, upon the deaths of every other Supreme Kai, became the Grand Supreme Kai), all five of them were much stronger than Frieza. In a quest to rule the universe, Bibidi unleashed Kid Buu on the peaceful Supreme Kai. Western Supreme Kai and Northern Supreme Kai were the first and second Supreme Kais to be killed by Kid Buu, respectively. Southern Supreme Kai attempted to defeat the Majin, but was absorbed, transforming Kid Buu into his buff Ultra Buu form.
Ultra Buu attempted to kill Shin, but was stopped by Grand Supreme Kai, who was absorbed by Ultra Buu moments later. Afterward, Ultra Buu transformed into the childish and fat Majin Buu, disrupting his reputation as a cold and efficient killing machine.
Dragon Ball Z Edit
During the Majin Buu Saga, Kibito transports himself, Shin and Gohan to the Sacred World of the Kais thanks to his Instantaneous Movement, so that Gohan can extract the Z Sword from its resting place on the Z Sword Plateau and train with it. Upon returning to Other World after his one-day pass to Earth had expired, Goku is able to sense Gohan's ki and travels to the Sacred World of the Kais using Instant Transmission after learning that Gohan is alive. During the course of Gohan's training, Goku and Gohan inadvertently release Old Kai from his imprisonment in the Z Sword. Old Kai then goes on to begin a long and drawn-out ritual that would unlock potential within Gohan.
After many hours, Old Kai succeeds in awakening Gohan's dormant powers. Kibito then returns Gohan to Earth so that he could confront Super Buu. When it becomes apparent that Gohan would be unable to defeat Super Buu (who had then absorbed Piccolo and Gotenks), Old Kai teaches Goku how to fuse using Potara Earrings. Shin and Kibito eagerly and successfully test this ability, fusing to become Kibito Kai. He is later appalled to discover that the Potara fusion is permanent, unlike the fusion induced by the Fusion Dance. Despite its apparent irreversibility, Goku feels that his fusing with Gohan is their only way to defeat Super Buu and returns to Earth with a pair of the Potara Earrings (however it would later turn out that Elder Kai was unaware that Potara fusion was only permanent if one of the fusee was a Supreme Kai in the manga or Shinjin in the anime. As a result, Goku and Vegeta's Potara Fusion Vegito only lasted an hour as they where both pure Saiyans).
Kibito Kai and Old Kai both continue to observe the unfolding of events from the Sacred World of the Kai, using a crystal ball.