Spirit World (霊界, Reikai), or Underworld in the manga, is the world where sentient beings (humans and demons) go after they die. This is not the equivalent of Heaven or Hell; it is sort of a station in between worlds where a soul's fate is decided. In other words, it is a junction at which numerous planes intersect, though it is more intimately connected with Human World than with Demon World.
The personalities of Spirit World's inhabitants are conducive to studying as much as possible about a subject and then acting on whatever choice seems to be the best option for a particular situation. This includes positive outcomes like restoring Yusuke Urameshi back to life when they couldn't decide his final place in the afterlife based on his last action before his first death, because said action greatly contradicted his observed personality, or negative outcomes like attempting to annihilate Yusuke once he was discovered to be a Mazoku descendant based on the potential damage he was capable of causing. Koenma, George, and Botan used to have these mindsets until they were all changed by Yusuke and as a result, they have stronger and more friendly bonds with others. Inhabitants include: Botan, Koenma, King Enma, George the Ogre, Ayame, Hinageshi etc.
This is where Koenma fills out paperwork to decide what the fate of a person's soul is to be. He judges the merit of their good deeds, weighs them against their bad deeds, and decides if they will go to Heaven or Hell, among other unstated possibilities. The palace is filled with ogres who do Koenma's grunt work and bring him a seemingly limitless amount of paperwork to do.
The main Spirit World vault holds three dark artifacts of terrible power, formed from existence itself to balance all of creation. The 1st artifact is the Orb of Baast, a orb that can steal a person's soul. The 2nd artifact is the Forlorn Hope, a magic mirror that can fulfill it's user's greatest desire on the night of a full moon at the cost of his or her life. Finally, the 3rd artifact is the Shadow Sword, a sword that can turn anything it cuts into the lowest class of Demon by a process called demonification. This vault is heavily guarded by a group of ogres, but they prove useless when Hiei, Kurama and Gouki raid the vault and subsequently "appropriate" the three items.
The supernatural video tape, the Chapter Black tape, was stored within its vaults before being taken by Shinobu Sensui and ultimately destroyed by Hiei.
- Main article: Hell
Hell (also called "Limbo" in FUNimation dub) is a possible destination for souls after death. The entrance looks like an old temple that has fallen into disrepair, with an ominous red sky behind it. This is the worst punishment a soul can receive, described as 10,000 years of everything (except your ability to feel pain) being cut away. The end result of this torture is a pitiful life, wishing it would cease to exist but knowing that it won't. Younger Toguro asked to be sent here after his death; he was originally going to receive a light punishment, but his guilt over what he had done in his life would not allow him to hear the pleas of others trying to convince him otherwise. Koenma finally agreed after realizing that he couldn't convince Toguro to change is mind.
The pacifier of Koenma, who is hardly seen without it. While such an accessory shows he is still a toddler in godly age, it is also an extremely powerful item within the ranks of Spirit World.
The pacifier itself is a medium for Koenma's own powers. Capable of creating barriers and resurrecting the dead through the collected spiritual power invested throughout the centuries from him, it also is the medium for Spirit World's most powerful defensive spell, the Ma Fū Kan (魔封環, translated as Demon Seal Ring), a spell which creates a powerful dimensional kekkai barrier, and is reserved when creation is facing catastrophic situations or is to naturally enter a much grim and darker period for its inhabitants.
King Enma's Golden Seal. Whoever holds the Seal controls the Spirit World.
A divine video cassette tape, created since the birth of humanity, and recorded within are the worldwide examples and events of darkness, cruelty, malice, evil, and ugly sin that mankind has committed in all of their existence, and is capable of committing.