Re:Zero IF

What IF are Web Novel chapters posted on Ncode Syosetsu each Aprils Fools at 1'o clock Japanese time. To celebrate Natsuki Subaru's birthday, the author writes a story which deviates from the original web novel. They are around four times longer than normal web novel chapters and in these chapters, it is shown what would have happened if Natsuki Subaru did something contrary from what he actually did. Each of the What IF stories resemble one of the seven deadly sins.

Re: Repeating a Life in Another World From Zero
Published on April fools of 2016, Tappei Nagatsuki wrote a chapter, commonly known as Kasaneru or Greed IF, where Natsuki Subaru accepted the contract which Echidna offered. The actual chapter starts after the fourth arc.

Re: Erring a Life in Another World From Zero
Published on April fools of 2017, Tappei Nagatsuki wrote a chapter, commonly known as Ayamatsu or Pride IF, where Reinhard didn't help Natsuki Subaru during the first arc with rescuing Emilia  from Elsa , what after 88 deaths made him fall into the dark side.

Re: Drowning a Life in Another World From Zero
Published on April fools of 2018, Tappei Nagatsuki wrote a chapter, commonly known as Oboreru or Wrath IF, where Natsuki Subaru accepted Beatrice's assistance against Ram during the second arc.

Re: Cobbling Together a Life in Another World From Zero
Published on April fools of 2019, Tappei Nagatsuki wrote a chapter, commonly known as Tsugihagu or Gluttony IF, where Murder became a habit for Natsuki Subaru, and started to read the victim's Book of the dead during the sixth arc.

Summary
Diverging from midway through Arc 6, after Natsuki Subaru decided to enact his plan to find out who his old self, "Natsuki Subaru" was by reading everyone's books of the dead, Subaru found himself in a remote region in Northern Lugnica after slaughtering a multitude of people who knew him on his journey to cobble the pieces of himself back together. In this remote region, Natsuki Subaru kept repeating a question to Amyu Sears (アミュー・シアーズ). He kept repeating "Hey, do you know my name?" to her. After some deliberation, the frightened young girl who was 14 years of age answered she didn't. Subaru, who was implied to have killed everyone else in her village, let her live.

Far prior to this event, Subaru stopped at the side of the main road to have lunch with Emilia, Beatrice, Ram, Julius Juukulius and Meili Portroute whom he had murdered back at the tower, and now appear as vivid hallucinations that speak to him quite friendly. After some lighthearted discussion with them about the perks of mayonnaise, all those individuals freeze in time, and fade from sight. In fact, Subaru only has one companion with him along the road, and that is Shaula.

As the two of them continue their journey down the main road, they meet Regin Suwen. After having realised that Regin knew the old "Natsuki Subaru", Subaru stopped to talk to him. Regin quickly realised that something was quite off with Subaru and suggested they go back to the place where he worked at to check if he was feeling okay, but before anything else could be said, Subaru commanded Shaula to kill him. She used her Hell's Snipe ability to burn off Regin's head, killing him instantly. After they hid his corpse, Subaru returned back to the Pleiades Watchtower to read his book of the dead.

Meanwhile, a short time after this, Echidna (Artificial Spirit), who had been allowed to escape from the tower by Subaru, made it back to Priestella. She revealed to the members present there that she was not Anastasia Hoshin anymore, and what had transpired back at the tower. Some of the members hearing this report don't believe her, but most of them do. After this, Echidna went to talk to Ricardo Welkin, who now had a metal claw-like appendage in place of his lost arm, about her future, and how she would stop impersonating Anastasia. As they talked, the city of Pristella was flooded by Subaru and both of them perished.

After the watergates were opened in Pristella, Otto Suwen and Garfiel Tinsel both survived the initial flooding. Suddenly, beams of light poured down upon them. With great difficulty, Garfiel managed to deflect them with his shields. Subaru finally arrived at the scene, and confronted the two, and had Shaula kill Garfiel, despite the latter still not believing that his former master had gone insane.

Otto is quick to understand what had happened, and tried to distract Subaru by telling him he should have sprung a surprise attack on Reinhard van Astrea first. Subaru shook his head at that notion and said that the reason he had flooded Priestella was to kill Otto himself. Everyone else was just a bonus. Subaru understood that it would take immaculate planning to even stand a chance at defeating Reinhard. As if affirming that, the hallucinations of some of the people he had killed affirmed Subaru's plan to himself, naming Otto as the most troublesome one. After that, Otto is killed by Shaula.

Shortly after Otto's death, Reinhard confronted Subaru and gloomily asked why he was doing this. Shaula immediately commented that Reinhard gave a very Reid Astrea-like impression, grasping his immense strength immediately. Subaru had come planned for this confrontation however, and he got Shaula to cast one of her Hell's Snipe towards the shelter which Felt was staying at. This allowed Shaula to pick Subaru up and escape with him from Priestella.

Later on, after having once again visited the Pleiades Tower to read everyone he had killed's books of the dead, Subaru continued with his journey to cobble himself together to get back the old "Natsuki Subaru".

Returning back to the events after Subaru had left Amyu alive, Reinhard found the girl alive and well, though shaken. Amyu had been left behind by Subaru to delay Reinhard, who was following his trail.

Meanwhile, Subaru was talking elsewhere with Shaula. He mentioned that eventually he will have to kill Reinhard or it would be impossible to cobble himself back together. And finally, after Reinhard would be killed, Shaula would have to be the last one to die. Shaula was extremely happy to hear her master say she'd be the last one to die, taking it almost like an "I love you". Subaru was surprised at her loyalty, and asked her why she kept on doing his bidding despite knowing she'd die, and she simply replied that it was because she loved her master. Subaru's plan was that after he gathered together all the pieces, he would die and fix things with everyone after he Returned by Death and got back his old self.

The scene ended on a reflection that the pieces that would need to be cobbled together were too many, and that even if he pieced them all back together his new form would not really understand his original one.

Re: Natsuki Rem Life in Another World From Zero
Contrary to all other What IF stories, the Natsuki Rem/Sloth IF was adapted into an entire novel. Occasionally, on Ram and Rem's birthday, the author releases a continuation for the What IF story.

The story is about what would have happened if Natsuki Subaru and Rem ran away from the Kingdom of Lugnica and abandoned everything behind in the third arc.

For more information, see Re:Zero IF.

Trivia

 * On April fools of 2015, the author wrote Butterfly's Dream / The Lust What IF chapter. However, he deleted it due to negative feedback, leaving behind only a message about its deletion.
 * The Butterfly's Dream is the first What IF chapter that was removed and deleted from the web novel site.