Death Note, Volume 12, page 188, has these two rules in regards to "Mu": "All humans will, without exception, eventually die." This is immediately followed by, "After they die, the place they go is MU (Nothingness)."
- English: After they die, the place they go is MU (Nothingness).
- Japanese: 死んだ後にいくところ、無である。
These rules can be applied to both:
A. Humans in general
B. Humans who have used a Death Note
The general implication of Mu is that all humans, regardless of their actions during life, simply cease to exist upon dying and are equal in death.
Mu is a Japanese and Korean word which denotes a negative: the absence of something. In Zen Buddhism, "Mu" is the answer given to an improperly phrased question that does not merit an answer.
The Japanese reads more like "after death, the place they go; it's Mu". And in Zen Buddhism, mu is an answer to a question that depends on invalid axioms.
So when the rule states "the place they go; it's Mu", it means the question relies on an invalid assumption. They don't go anywhere, because there is nowhere for them to go to. This is why it gets anglicized as "Nothingness", because all that exposition is way too much detail for a notebook rule.
Tsugumi Ohba, the author of Death Note, was influenced by Buddhism; this is why the series has 108 chapters. This matches up with various things Ohba says about life after death in Death Note: How to Read 13:
- For me, one of the premises of the series is that once a person died, they could never come back to life. I really wanted to set a rule that bringing characters back to life is cheating. That's why death equals "nothingness" ... If I had to choose [a theme to express throughout the series], I'd say "Humans will all eventually die and never come back to life, so let's give it our all while we're alive".
Death equals "nothingness" だから、”無”なんです--"so, it's Mu"
The number 108 represents the 108 earthly desires in Buddhism.
Mu as Hades/Underworld but not Hell
Hades is regarded as a plane of nothingness, or the void. Hell is regarded as a plane of suffering, usually yielding inhabitants guilty for their sins. In various ancient mythologies, the souls of the dead are drawn to a plane of existence, or the spirit world. In Biblical scriptures, Sheol is described as such a place where nothingness lurks, contrasting it with Gehenna, a plane of existence where God sends the wicked people to reside.
In Buddhism, such an interpretation of Hades/Underworld may be analogous to Nirvana, or an empty plane which is a principal of Zen Buddhism. As Nirvana is usually interpreted as free from the physical world, it may be the same as the primordial plane that existed before and exists between the physical dimensions.
The underworld is sometimes represented with a chaotic nature, describing the place and its inhabitants as dangerous and monstrous figures, because the realm skews its way from the stable laws in the physical world. Such a description reflects the grotesque nature of the shinigami world.
In Judeo-Christian theology, the differences are noted between Shoel and Gehenna, where Shoel is described as a more neutral realm, however have been mistranslated as Hell in the King James translations. It is generally agreed that Shoel and Hades is the temporary place for the dead, before being resurrected on the final judgement.