This is my first fanfic so I hope you like it. I have cross posted it on AO3.

This fanfic was inspired by another one called Back to Zero (the part about the cure at least)

Everything was right in his life, he had friends, both old and new ones, his family was becoming once more united, he had a job he loved, a job guiding the youth of tomorrow like his late teacher, maybe even the beginnings of a relationship. Nagisa was truly happy and many times he thought that nothing could go wrong. Oh, how mistaken he was...

It all started one seemingly normal night, when he got a phone call. He groggily sat up from the bed and walked to the phone wondering who it might be at such a late hour. He picked up the phone and answered, not knowing who it was because of the unknown number.

"Hello? It is kinda late so if the matter is not important can it be left for tomorrow?"

A voice came from the phone, one he hadn't heard in a while. "Oh, sorry Nagisa I am currently in a foreign lab, so I forgot it would be night in Japan, I just wanted to tell you of a discovery I made." Nagisa instantly recognized that voice, it was Okuda.

"Okuda! I haven't heard from you in a while, it's nice to talk to you again, how has life been treating you?" Nagisa quickly answered to dissipate any possible unease.

"I've been well. I've visited a lot of interesting places all over the world, it's amazing some of the things people can do with chemistry. Anyways, you were the one closest to sensei so I thought I should tell you this in person." Her voice sounded sad, and a bit nostalgic. "Recently I've discovered a certain substance that would have fully stabilized Korosensei and, despite it being too late to do anything for him, the rest of our class decided to hold a little ceremony for sensei, where we would offer him a little tube with the substance and maybe some things relative to our current life as a little token of appreciation for the things he helped accomplish. I wanted to invite you to come too."

It had been as while since He'd truly had more of a passing fond thought about Korosensei. Actually, it made him realize how much he had been avoiding the topic, always steering the any conversations away from it. He decided to face sensei at least once more so he too could show his thankfulness. "I'll go, Korosensei deserves a lot of respect for what he for us, I'll see you there Okuda."

"Ok, I'll tell the others you'll be coming, the ceremony will happen in the old classroom, in 2 days in the afternoon. See you there Nagisa, Goodbye!"

"Goodbye" And he hung up.

Nagisa lost himself in his thoughts. 'I need to find something that Korosensei would like... What can I bring that has anything to do with being a teacher that can be considered special? I'll think of something tomorrow, it is too late for anything like that.'

He went back to bed, laid down and slept. And for the first time in years, his sleep was not peaceful.

-Nightmare-

Nagisa looked around, confused about his current location. He appeared to be in his old classroom. An eerie red light was coming from the windows and the air was heavy.

Wanting to find out what was happening, Nagisa walked to the door and opened it. He was instantly blasted by a strong gust of wind and slammed on the wall opposite to the door. He then started hearing footsteps. He tried to move but found himself paralyzed, vulnerable and a shadow started forming on the doorway, a tentacled shadow. He was feeling more and more frightened every second that passed, wondering what the hell was going on in this place. Then a figure appeared at the door and Nagisa was petrified. There stood Korosensei, a hole stained with fresh crimson blood in his chest, a cruel smirk on his usually friendly face, his entire body was black and contorted and in his hand was an Anti-sensei knife, covered in blood. He started walking towards Nagisa.

"You killed me, you were the one who did this, you are the one who killed the teacher all your classmates loved." Korosensei's voice was distorted, almost demonic.

"Y-you were the one who asked me to! You were the one who TOLD me to do it!" Nagisa answered, frightened.

Korosensei didn't appear to even register what he said, simply chuckling, while slowly approaching Nagisa. "Nurufufufufu, you could have saved me Nagisa-kun, there was a cure, but you still killed me"

Nagisa started feeling guilt, crushing guilt, combined with dread because of what Korosensei might do to him. "We didn't know it was possible to cure you and you were already dying! How could we have saved you!?"

"You killed me Nagisa, and for that I will take your life, Nurufufufufufufufufufu!" Korosensei's voice turned even more demonic, if at all possible and his laugh became malicious. He then plunged the knife into Nagisa's chest and though it was an Anti-sensei knife it still cut through him and hit heart, still laughing while doing it.

Nagisa felt an overwhelming amount of pain, and he screamed for what seemed like hours before he was rudely awoken.

-Real World-

"-gisa? Nagisa! Are you ok?!" He heard a feminine voice say.

"W-what? Where am I? What happened?" He asked still dazed by the pain he felt in the dream.

"You were screaming and struggling in your sleep so I tried to wake you up, but you were moving around too much." Nagisa finally connected the voice to Akari. He had forgotten that she was coming today to hang out with him. After that year they found themselves separated for a while but a year ago he moved into the same town as him and they rekindled their friendship and what was becoming recently a relationship.

"Oh, I just had a nightmare, nothing much. Just a normal nightmare." He tried to clear her worry, but she noticed the tired tone of his voice.

"Nagisa, you very rarely ever have nightmares, and even then most nightmares don't have people thrashing around, screaming and sweating! Don't try to dismiss this as nothing." She sternly told him. "What was it about and what made you have such a nightmare?"

"I really don't want to talk about it Akari, please." He asked in a pleading tone.

Noticing how uneasy he was, she decided to concede. "You are making me worry Nagisa, but for now I will leave it at that. But remember, if it persists I will not leave you alone."

"Thanks. Hey, did Okuda tell you of the ceremony?"

"Ceremony? Ah, the one to honor Korosensei's memory. Yeah she did, I'm going to bring one of the props from my newest drama."

"The one about the two enemy assassins who fall in love right? That's a great idea. I still have no clue what to bring..."

"I'm sure you'll think of something to bring. Now let's go to that new park, it's a bright day and the fresh air will do you some good."

-2 days later - Class E's classroom-

Everyone was at the ceremony. Karma and Okuda were already there when Nagisa arrived with Akari, as were Karasuma-sensei and Irina-sensei while Ritsu was in a computer screen. The rest of the class quickly arrived, Chiba and Hayami came after them, after that came Itona together with Terasaka and his gang, and the rest of the class soon after.

Karasuma-sensei opened the ceremony with a small speech about the, as he said 'Octopus', and then everybody gave their own two cents. One by one every student went to the front of the class and spoke to the others, leaving Nagisa for last.

Nagisa slowly walked to the front and faced his ex-classmates, his friends with who he spent an amazing year. "The year we spent with Korosensei was the best year in my life." He started. "He believed in us and helped us when no one else would, he made sure we had fun and catered to our needs, he was the best teacher we could have ever wished for. The only thing I would have desired of him that we didn't get was his survival. The only thing that could have made that year better was if Korosensei had stayed with us, so we could talk to him and seek his advice whenever we felt lost. That is my only regret, that I didn't manage to save Korosensei." The air was heavy and gloomy, everyone felt regret for their one failure.

"Ok, everyone, it's time for the offerings we brought for Korosensei." Irina-sensei said, trying to lighten the atmosphere.

Everyone left the classroom, to a small shrine built on the forest that covered the mountain. One by one, they all approached the shrine and left an object which they felt best represented their appreciation for their late teacher. Sugino left a baseball he signed himself, Akari left a small prop knife and a bit of pudding, Karasuma-sensei left an Anti-sensei gun, Okajima left a bunch of adult magazines, and slowly everyone left a little offering. In the end came Nagisa and Okuda, she left the vial with the substance which could have saved Korosensei and Nagisa brought forth his offering, a little notebook with the names of every student he had taught up to the time he wrote it.

After the ceremony a few more words were spoken between the students before they all went their ways, the mood sullen.

Karma though, he found it to be the best opportunity to prank Nagisa. He approached him slowly while his back was turned to him and poked him in the back with something. "Ow, what was that for?" Nagisa asked while turning around to see what had happened.

"Just trying to do something nostalgic, too bad it doesn't work on people." Karma said with a mischevious tone.

As Nagisa turned around he spotted what he ha been poked with, an Anti-sensei knife. The knife brought him flashbacks of the nightmare 2 days before and he started panicking. It's your fault, Nagisa, your fault!You could have saved me but now you have my blood in your hands, it is YOUR FAULT! Nagisa started hiperventilating much to Karma and Akari's concern and then passed out.

-The day after - Hospital-

Nagisa woke up somewhere unfamiliar, white walls, white sheets and constant beeping. Looking around he realized he was in an hospital, he tried to remember what happened and realized he had a panic attack. He would have a lot to explain to Akari.

Paying a bit more attention to his surroundings he noticed that someone was sleeping in a chair next to him, which he realized was Akari. He watched her sleep for what felt like an hour before she started stirring. The first thing he noticed were the bags under her eyes and her look of concern.

"Nagisa! Oh God, I was so worried, you just started to hiperventilate and then you collapsed, so I and Karma were trying to see what was wrong, but we couldn't find anything wrong so we called the hospital and they sent an ambulance and they told us you had a panic attack but-" Akari started before being interrupted by Nagisa.

"Calm down, you're speaking too fast. Just calm down, I'm ok." Nagisa tried to reassure her.

"Ok?! OK?! How can you tell me you are ok when you just passed out! What happened?"

"As the doctors have told you I had a panic attack, seeing that knife in Karma's hands just made me remember something unpleasant..." Said Nagisa, sounding a bit vague.

"It has to do with your nightmare doesn't it? The one you refused to talk about." Asked Akari concerned.

"Yeah, it does. It was a very bad nightmare."

"Do you want to talk about it? Y'know, to get it off your chest?"

"I-I, I had a nightmare in which I was trapped in our classroom and then Korosensei slowly approached me, looking terrifying. He blamed for his death,told me that it was I who killed me, who failed to save him. And I was just paralyzed as he stabbed me in the chest with one of his knives over and over again..."

"Nagisa, you know Korosensei would never blame you. There was no way to save him and he knew it, he was the one who asked you to do it. And just as he doesn't blame you, neither do any of us." Akari tried to make him feel less guilty. "Okuda even left me a vial of that substance she made so I could give you as a memento." She said,showing him a vial of yellow liquid.

"Thank you Akari, I know no one would blame me but subconsciously I still blame myself. But I'll be ok, just need a bit of rest." He told her reassuringly and then added in a whisper. "I hope."

"That's good, I've got to work so I'll be leaving now, see you later Nagisa." Akari told him, leaving the vial on the table next to the bed.

"Goodbye Akari, break a leg."

Akari left, leaving Nagisa alone with his thoughts which were becoming more and more grim.

-3 months later-

Akari din't know what to do anymore. Since that day in the hospital Nagisa had become less and less like the person she knew. A few days after the hospital he started looking sick and tired, with bag under his eyes and a sullen mood whenever he went. Then he stopped leaving his home unless it was for school or absolutely necessary, even his students noticed something was very wrong. He took a long sick a month and a half after leaving the hospital. The straw that broke the camel's back was when she found him draped over his couch with a few empty beer bottles on his table. She already knew something needed to be done but that scene made her decide to take an active role in helping him. She asked Ritsu for help, because of her easy access to information all over the world and they spent a month trying to getting him back to how he was. Gifts, counseling, emotional support, dragging him out into the sun, group meetings, even talking to his other friends, nothing seemed to work. Nagisa just fell deeper and deeper into his hole, closing himself off and trying to burn the bridges between himself and everyone.

It was on one of the days he left his house to drink himself under in a bar as he had run out of alcohol at home that everything changed. Akari had noticed he wasn't home and went searching for him in the usual bar. She had been talking to Ritsu, trying to find any way to bring him back after so many failures. She went into the bar and found him together with four glasses of something alcoholic.

"Nagisa! You have to stop this! You're going to end up killing yourself." She tried to reason with him.

"I-I-I k-killed him so it-t's only natural that I e-end up dying." He answered, clearly drunk.

"Don't ever say that Nagisa! Everyone would miss you if you died."

"Lies! T-there is nobody who would f-feel sad if I died, everyone b-blames me for his death."

"No one does! You're being unreasonable, Nagisa. Come on, we have to get you home." She insisted grabbing ahold of him and pulling him out of the bar after paying his tab.

They were crossing the road in front of the bar, Akari supporting him while Ritsu tried to comfort him when it happened.

Nagisa first saw white light, then he felt lot of pain, red mixed with green, broken electronics, then black and finally nothing. He believed he would never wake up again, his only regret was having taken Akari and Ritsu with him. He would have never guessed that his new mission was just about to start.