{This outline was envisioned and made in around an hour. I plan to turn this into a ~15k words novellete}

Prologue

"12 days, 23 hours, 45 minutes, 36 secon'ds. That's what 12.99 days amount to".

After Kira is defeated the SPK carves a deep hole in the ground at a secret location and throws the Death Notes into it and fill it with concrete as their last act before disbanding. With Kira no longer existing crime starts to rise again and ramps up until decades later a nuclear war takes place. Very few survive and there is no law anymore. People live underground tunnels in isolated small tribes because of the radioactive atmosphere at the surface. A tunnel digger named Yami, sumbles upon a concrete wall, breaks it and finds a Death Note. It is the one with the fake 13 days rule.

Chapter I

Yami isn't a killer but is being threatened by a gang within his tribe. The gang forces the tribe to give them more food and supplies than it would be fair to give. Yami and ponders for days whether to use the note or not. He refuses to kill anybody other than the gang so he thinks he will die 13 days after killing the last enemy if he choses to use the note. The 13 days rule is crossed out but he takes it for true anyway out of precaution.

He decides to use the note by forcing himself to believe he will have the courage to allow himself to die after 13 days counted from his last kill.

Chapter II

Yami kills one gang member every 12.99 days. After the last enemy is killed he becomes ultra-anxious. His supposedly remaining 13 days of life seem to pass ultra-slowly rather than faster... Every hour he looks at the note and feels an urge to protect his own life by keeping writing names on it. But he knows of nobody else who'd deserve to die; after the nuclear apocalypse people live in isolated small tribes and Yami's tribe is already cleansed of all evil people.

Chapter III

Yami then gathers provisions planning to set out to find another tribe and cleanse it. He grabs a hazmat suit and asks Keikai, the tribe's leader to let him out of the bunker under some excuse. Keikai doesn't see any reason to object. The exit door has a two man system, meaning there are two handles away from each other that must be pulled at the same time for the door to open. Yami and Keikai go to the exit door and each pulls one handle, in sync, and the door opens. Yami then says goodbye to Keikai and moves into the surface in the hazmat suit. For 12.99 days he roams but finds nothing. Desperate he fails to accept death and writes a name of an innocent person, Kodai, the oldest from his tribe. But a few days later the guilt is so overwhelming he decides to kill himself. He thinks of going back to the tribe to apologize to Jundo, Kodai's brother before killing himself but his shame won't allow that either. Failure to gather forces to apologize and to kill himself makes Yami's guilt even stronger.

Chapter IV

Crestfallen, Yami then goes to back to his tribe's bunker. It takes another 12.99 days to return. At the bunker's entrance Yami picks up his death note and prepares to write something in it. He then writes: "Yami. Reveals the existence of the Death Note to Jundo, confesses his killings, apologizes, goes to his room and hangs himself".

Yami reveals the existence of the Death Note to Jundo, confesses his killings, apologizes, goes to his room and sets the rope. But strangely he feels no courage to hang himself and panics. "What has gone wrong?!" - He screams - "Why can't I do it?!". Keikai breaks into his room. Yami sees another Death Note in Keikai's hands and his face goes pale. Other people barge into the room after Keikai. Keikai tells Yami and everyone in the room that after Yami disappeared he started investigating and found the carved concrete wall. He says he carved further and found another death note and realized what had happened. He says that when Yami came back he saw him opening his note, and fearing Yami would take yet another life he quickly wrote Yami's name on the death note he had found. Yami then, on the verge of a nervous collapse, sifts through the rules written in his note and finds the one he was looking for:

"If the same name is written in two or more Death Notes within 0.06 seconds, the entry is regarded as simultaneous; the Death Notes will not take effect and the individual will not die."

Chapter V

Yami's eyes shrink into trembling dots and Keikai and other tribe members rush to him and tie him up with the very rope he planned to hang himself with. Yami is taken to a cell and untied. For the first hours Yami fears he will either be lynched or die at any moment since it's been very close to 13 days since he last used his note. But the 13 days complete and he is still alive. He realizes the rule was fake and that the crossing the rule had is now a definite proof it is fake. Guilt still consumes him but after sometime he realizes his apology to Jundo and the courage to face him were legit since he was after all not being controlled by the death note since him and Jundo had written 'Yami' on their death notes simultaneously. Staring at the damp oil lamp at the ceiling near his cell Yami's eyes fill with tears and he feels prepared to accept whatever fate the tribe will lay unto him even if its a death sentence.

Chapter VI

A day later Jundo goes to Yami's cell and faces him alone. Yami looks silent at Jundo. Jundo also looks silent at Yami. They stare at each other worldess for a minute. Keikai appears and stands by Jundo's side. Both stand close to Yami's cell's grill. Keikai has the two notes in his hands. Ryuuk appears. Yami says he forfeits his note. Ryuuk extends his hands to take the notes from Keikai. Noticing Keikai didn't forfeit his note, Yami unexpectedly jumps and kicks the notes from Jundo's arms into the lamp. The notes burn to ash and Yami thinks with satisfaction that him and Keikai will die. Ryuuk, seeing there is nothing more for him to do there, nods in a laughter and goes away. He says before leaving: "No one here will die. And you won't forget anything either, Yami, since you touched the notes again when you kicked them". he then raises his finger in a scolding manner and says: "Mankind is no longer interesting, you know, you ruined the juiciest apples in universe". Ryuuk fades away forever from the human world.

After another long silence, Jundo opens his mouth and forgives Yami, but Keikai still condemns Yami to live in that cell forever. Yami lowers his head showing respect for Keikai's judgement, fully accepting his fate.

Epilogue

Jundo and Keikai leave Yami to his solitude and Yami looks up to the lamp in the ceiling and mutters, with great depression: "This is my new sunshine forever... But hasn't it been this way already since a long time? For me and for everyone else in the world; this is the only sunshine we will in fact ever ever have".