For the genuine rules, see Rules of the Death Note.
The Fake Death Note Rules are two rules that Light Yagami persuades the Shinigami Ryuk to write into the Death Note as part of a plan to clear him of suspicion. These rules ultimately lead to his downfall.
- If the user of the Death Note fails to consecutively write names within 13 days of each other, then the user will die.
- If a person makes this notebook unusable by tearing it up or burning it, all the humans who have touched the Death Note will die.
When Misa Amane is taken into custody on the suspicion of being the Second Kira, the Shinigami Rem tells Light that he has to free her or else Rem will kill him. Light himself knows that it is only a matter of time before L arrests him on grounds of being the original Kira. Light therefore comes up with a plan which will clear both his name and Misa's.
As part of the process, Light has Ryuk write two extra false rules in the Death Note alongside the genuine ones. He then entrusts this Death Note to Rem and instructs her to pass it on to another unknown human.
Light surrenders himself to L and the rest of the Japanese Task Force and is incarcerated. He and Misa are kept isolated in single cells with their hands bound. Meanwhile, Rem lets another human take possession of her Death Note on the condition that he resumes Kira's killing of criminals as well as furthering his own ends.
Light and Misa renounce their ownership of their respective Death Notes, causing them to lose all memories of ever using them and of being either Kira or the Second Kira. They now genuinely believe themselves to be innocent of any crimes, and after nearly two months of incarceration, their jailers are persuaded to release them. As Light had planned before relinquishing the Death Note and his memories, they both join forces with L in order to track down the new Kira. This turns out to be Kyosuke Higuchi, who is arrested and his Death Note seized by L, Light, and the police. Once he touches it, Light recovers all his memories of being Kira and sets into motion the next stage of his plan: the killing of his nemesis, L.
The "13 day" rule goes further in establishing Light and Misa's innocence in the eyes of the Kira Task Force since they had been held in custody for over 50 days with no possible access to the Death Note, while the other rule makes the police reluctant to destroy the Death Note since many of them had touched it as well.
Misa later regains her own memories of being the Second Kira by recovering a second Death Note that Light had hidden. She uses it to resume the killing of criminals. When crooks again begin dying of heart attacks, L notes how soon this is after Misa's surveillance is lifted.
L is not convinced that the "13 day rule" is genuine since there is still plenty of other evidence pointing to Light and Misa being the two Kiras. He thus contacts his associate Watari, charging him with finding two Death Row inmates who could test the rule: one would write down in the Death Note the name of the other, who would die, and they would be able to see if the writer dies after thirteen days or not.
Rem realizes this would expose the rule as a fake and that Misa would again be the focus of the investigation, destroying what life and happiness she had left, having twice made the deal for the Shinigami Eyes—once with Rem and once with Ryuk—and halving her lifespan on both occasions. Rem therefore uses a third Death Note in order to kill L and Watari, but as she purposefully extends Misa's life span, this also causes her own death. As a result, Light is rid of both L and Rem, the greatest obstacles in his path toward a new world order.
The rest of the Kira Task Force had opposed L's method of testing the "13 day rule and," following his death, keep the Death Note under lock and key without either testing or destroying it.
In the long term, however, the "13 day rule" proves to be a factor in Light's undoing as L's successors, Mello and Near, discover for themselves that it is a fake and pass this on to the Task Force, planting seeds of doubt about Light, who is now leading the group. Shuichi Aizawa and Kanzo Mogi in particular begin to again suspect that Misa and Light may have been the original Kiras, even though they do not want to believe that Light is Kira out of respect for his father Soichiro Yagami, a much-respected superior who is killed in a confrontation with Mello for which Light is ultimately responsible.
Aizawa contacts Near and passes on information about the Death Note that Near was previously unaware of. When Light is finally exposed as Kira and killed, Ryuk confirms that the two rules are false, and Near is thus able to destroy the Death Notes.