This takes place right at the end of Arc 6 (to be more exact, in the last chapter of Arc 6) so spoilers ahead.
Prologue:
"Shit...I've come this far...!"
"Barusu!"
Subaru ran towards Rem and Louis in an attempt to save the two only to be consumed by the shadows covering the area. As things were not going well for the boy, he started to say,
"I want to overdo it..."
"-I love you. I love you. I love you. I love you. I love you."
"I'm sorry, but I can't answer that...Now that word is a landmine to me. I just missed the hand of the person who said that."
"-I love you. I love you. I love you. I love you. I love you. I love you. I love you. I love you. I love you. I love you. I love you. I love you. I love you. I love you."
"... Is it like each other who don't have ears to hear? Then, just drink it up."
There was no hope that could come to this empty space and live from here. Natsuki Subaru would die mercilessly in this darkness.
"Once back, the worst may be waiting for you. Louis, who has a clean head, may now try to take away return by death."
"-I love you. I love you. I love you. I love you. I love you. I love you. I love you. I love you. I love you. I love you. I love you. I love you. I love you. I love you."
"But I won't lose. I won't lose. I will fight again and again. I will fight again and again. This time, I will keep my promise."
"-I love you. I love you. I love you. I love you. I love you. I love you. I love you. I love you. I love you. I love you. I love you. I love you. I love you. I love you."
"-I will fight many times tomorrow."
'If you love me, you won't be crushed by the words that are repeated so many times. I'm sorry, but if my heart was hurt by that word, it was completely hurt just before.'
With such love, Natsuki Subaru cannot be tied up however the repeated words of love do not even care about Subaru's refusal.
Indeed, blind words of love rushed to fill the world itself, and Natsuki Subaru's existence was swallowed into the darkness.
"-I love you. I love you. I love you. I love you. I love you. I love you. I love you. I love you. I love you. I love you. I love you. I love you. I love you. I love you. I love you. I love you. I love you. I love you. I love you. I love you. I love you. I love you. I love you. I love you. I love you. I love you. I love you. I love you. I love you. I love you. I love you. I love you. I love you. I love you. I love you. I love you. I love you. I love you"
Just as all seems lost, a voice speaks out,
"-I, Volcanica. By the old pact, the aspirations of those who reach the summit are questioned."
The next moment, the tremendous blue light shining down from directly above hit the darkness surrounding the world.
And the fierce light eats up the darkness, and the world changes color at once.
As Subaru's mind was in a dark place, all the boy could think was how he was going to stop Louis and save everyone from the problems of the Pleiades Watchtower.
Eventually he was able to wake up, with his mind a bit disoriented due to having been consumed by Satella's shadow alongside a sleeping Rem and a sleeping Louis.
The boy looked around and was a bit confused on where he was since the last place he thought he would be was in the Pleiades Watchtower yet he was in some dark place.
"Satella?"
The boy looked around as he called the witch's name yet he heard nothing but an empty void. This caused him to worry that he might have died permanently. He got up and took a moment to think about his situation.
"Where am I?"
He walked around for a bit, wanting to see if he could find something only to realize that he wasn't going anywhere. This caused him to just sit down and rest for a bit, getting his thoughts together.
'Okay. Okay. Let's see what happened. Shaula is gone. Louis didn't die, but why was she in the real world? That's a question for later...shit! Rem!'
Subaru jumped back up once he recalled that Rem was also covered in shadows, causing him to run around the dark void in search of her.
As he was running around, his mind was also thinking about Emilia, Beatrice, Ram, Julius, Anastasia, and Meili.
He doesn't know what happened to them or to anyone for that matter but he is extremely worried for them.
'Emilia-tan...Beako...Ram...Julius...Anastasia...Meili...Rem...I hope you are all okay.'
He ran and ran for a while yet all he saw was a dark road ahead.
"Where the hell am I?"
Subaru says this with an annoyed tone of voice, not sure where he is at or where he is even heading towards.
He stopped running and just decided to sit down for a bit, trying to think of the situation he has landed himself in.
He looked up at the dark void with his mind thinking about all the failed loops he had watched in the second trial.
Not only those but also the other deaths and how that might have impacted the worlds he left behind.
"Am I...in hell?"
"Not quite."
"What?!"
Subaru jumped back up on his feet and looked behind to see who was the second voice speaking to him.
As he looked behind him, he noticed that nobody was there except the dark empty void that he has been seeing for a while now.
"Great, I guess I'm going insane now, just like the guy from Cast Away now. All I'm missing now is a ball."
"You are not going insane."
As the voice finished speaking, a dark figure of a human started to form. Subaru got himself ready to either fight or run, unsure on whether this thing is even a friend or a foe.
He waited for a moment before finally the dark figure had a human form. Once in human form, the two could have a casual conversation.
"Who are you?"
"Let's just say I'm a watcher and I've been watching you, Natsuki Subaru."
"For what reason?"
"It's just my job."
"What a lame job."
"Says the one that has died multiple times."
"...wait what?"
Subaru's face went pale the moment his mind managed to process whatever the person in front of him said. He touched his chest for a moment with fear yet nothing happened.
The witch's shadow hand didn't come for him like usual. The dark human approached Subaru and patted his shoulder while saying,
"No need to worry, Natsuki Subaru, The Witch of Envy or better known as Satella can't reach you here...nor does she know you are here."
"What do you mean?"
Subaru said this as he took a step back, wanting to be away from the dark person right in front of him. As he does this, the dark figure of a human tries to calm the situation down.
"Now Now, no need to be hostile-"
"Just stop, tell me who you really are? What do you know?"
"Okay Okay, no need to be hostile. My name is not of importance for now but you can call me the watcher."
"The watcher? What kind of name is that?"
"Anyway, I have been watching all potential timelines and you are the one I was hoping to speak with."
"Why? Also timelines? Wait...does this mean that-"
"Don't ask, I won't be giving you answers regarding that but there is a reason why I brought you here."
"Why?"
Subaru crossed his arms as he looked at the watcher with a serious and curious facial expression, a bit curious to know what exactly he was doing here.
The watcher turned towards the empty void and with just a snap of his fingers, he managed to bring light to the place they were at.
As the dark void became a light room, Subaru was completely surprised by this.
The watcher noticed this and smiled before forming himself into an actual person.
The form he took was an older Subaru, catching the real Natsuki Subaru off guard.
"What the hell?!"
"I'm just taking the form you can be accustomed to."
"Why not just be yourself? Why do you have to look like...an older me?"
"I mean, I would show you my real form but I would rather not."
"Why?"
"Not important, I'm just here to ask for your assistance."
"In what?"
Subaru was very skeptical when he heard this, not trusting the person looking like an older him.
Before he could say anything, the watcher snapped his fingers once again to change the room into Lugnica yet the kingdom was on fire.
Subaru's eyes turned from one of skepticism to one of despair as he saw Lugnica in flames.
"Wh-What's going on?!"
"A world where a boy goes mad, takes the wrong path and goes to great lengths to save someone who doesn't even know who he is...well, sees him as a villain."
"Wh-What are you talking about?"
As Subaru said this, the room changed into a castle, one that Subaru isn't familiar with.
As he was thinking how unfamiliar the place is, the watcher continued to speak.
"A world where he doesn't trust anybody and ends up creating a dark reputation for himself."
"What...?"
Subaru looked with uncertainty since he doesn't know what he is looking at.
While he was confused, the watcher snapped his fingers once again to change the scenery to one of a japan looking place.
"Where are we?"
"You are looking at a world where a boy abandoned everyone with someone he cares about, making a life for himself."
"..."
Subaru just stood silent as he heard what the watcher was saying to him, beginning to think that all of these events he is seeing are from the failed loops.
Considering that most of the loops involved Emilia dying in some way, which would cause Puck to do some damage before Reinhard takes him down. As Subaru was thinking this, the world shifted to one of familiarity.
"The mansion?"
"A world where somehow the boy was able to get all the girls...don't know how but he did."
"What does that supposed to mean?"
"Don't think about it as much, just a stupid world."
"...Okay."
Subaru was just left confused by the statement the watcher said to which the watcher quickly snapped his fingers to change the world again.
As he switched the world, Subaru recognized the place as it was the mansion of the Miload family.
"What are we doing here?"
"This may look familiar to you, Natsuki Subaru, but this world tells a different story, one of a boy who had good intentions and is trying his best to save everyone but chose the wrong person to help him. Making him live a never ending nightmare."
"How...?"
Subaru was left confused by this since he doesn't know how the current Emilia camp's mansion could be in a different world.
As he was thinking on the words of the watcher, the room changed again to the Auguria Sand Dunes, surprising Subaru.
"The Pleiades Watchtower? What are we doing here?"
"Well, you see, this is a world where the boy is trying to collect the pieces of the person he once was, not realizing the damage he really is causing."
"Wait, the boy trying to...is that boy me?"
Subaru said this with worry in his eyes, starting to see that this could be the case. The watcher looked at the boy yet he didn't answer the question.
All the watcher did was snap his fingers to return the room back to the white room it once was.
As they returned back there, Subaru grabbed the watcher's shoulder as he prepared to punch him.
"Answer the damn question!"
"Hm. Violence is not going to get you shit, Natsuki Subaru."
"Then why do you keep avoiding the topic?!"
"It's not something you need to know, kid! Just trust me on this!"
Subaru looked skeptical and slightly angry towards the watcher yet he managed to calm down after taking a deep breath.
Once he was calmed down, he looked at the watcher with a serious facial expression before saying,
"Okay, why am I here?"
"Well I need your help to follow these fallen worlds and at the very least set them straight...well the best you can anyway."
"Okay...you do realize I have no strength or powers, unless you give me some."
"Hm. No. But there is a way to make sure you don't end up dying on me."
"Okay, how? Also I can't die, as you can tell."
"That is true, you can't die in your world but in these worlds, you die and that's it."
"Are you serious?"
"Well...if the person with Return by Death dies in that world, you'll also experience that without dying but if the world completes and you are dead, then well...you will permanently die."
"What?"
Subaru was dumbfounded by this since on the bright side, he doesn't have to experience death per say though on the bad side of things, he would have to avoid dying.
He crossed his arms and thought about this for a moment before he looked at the watcher.
"Why me?"
"Hm?"
"Why not Reinhard? Emilia-tan? Beako? Ram? Garfiel? Maybe Otto? Hell, even Roswaal the prick would be able to do more than me."
"But I choose you. Once you go and see these worlds for yourself, you'll understand why I chose you."
The boy stood silent since he doesn't know how to respond to this, knowing that the only good thing he could probably do is use Invisible Providence.
Though Invisible Providence would only work once, making him useless once he uses it.
'Damn, I wish I could still use Shamak.'
"Alright, well is there anything else I should know, Watcher?"
"Nothing really, that's all you need to-"
"I refuse."
"What?"
"You heard me, I refuse."
Subaru crossed his arms and started to walk away, catching the watcher off guard.
"Hold on, where the hell are you going?"
"I'm going to find my friends. There is no need to stay here."
"What?!"
Subaru continues to walk away, not wanting to deal with someone that could get him permanently killed or in Satella's bad side.
As he was walking away from the watcher, the watcher just looked at the boy with an annoyed facial expression.
He took a moment to think of a way to get Subaru back on board with his plan until it hit him.
He quickly snapped his fingers to appear in front of Subaru. As he was in front of the boy, he began to say,
"Fine Fine, what do you want?"
"A chance to survive. I'm not going in there and getting myself killed for some cause I don't know of."
"Okay, fine. Considering that you are weak and probably would die very quickly in some of the worlds I would be sending you, I'll make you a deal. For certain worlds that require some muscle or strength, you'll have some backup."
"Backup?"
"You'll see when you enter the world."
"Will they be actual allies? Cause otherwise-"
"Yes! It will be people who will fight by your side! I'm not going to have someone like Petelguese fighting by your side or Sirius."
"Petelguese and I would not get anything done and Sirius...I rather not. And you better not put Regulus' or else-"
"I get it, Natsuki Subaru!"
"Okay...wait why can't I choose my allies?"
"Because I have a gut feeling that Reinhard would be the obvious choice for almost any situation you would be put in."
"I mean you aren't wrong..."
"Anyway, let's just get this going. The more time we waste, the less time you'll have in saving these worlds."
The Watcher snapping his fingers again to make a door appear near the two. Subaru turned towards the door and recognized that it was one of the symbols of sin.
'What could this mean?'
Subaru was trying to understand the meaning of this yet he ignored it and just opened the door.
Before he went through the door, he looked at the watcher and had one last thing he had to ask,
"I doubt you would be honest in this but are you a threat to me?"
"...no, I'm not. I am aware that you've asked this to Roswaal L. Mathers in a failed loop and he wasn't 100 percent honest about the question but I really am not a threat to you, Natsuki Subaru."
"Then what is your purpose in sending me to these fallen worlds?"
"To get them back on track. A few have not fallen but you need to make the person with your ability to find some peace."
"...are the people with Return By Death...me?"
"...You'll just have to find out for yourself, Natsuki Subaru."
Subaru looked at the watcher with a serious facial expression before going into the door, a bit curious yet nervous to see how things would go for him.
As Subaru left the white room, the watcher had a smile on his face before changing from his form of an older Natsuki Subaru into his true form.
He wore normal human attire, had short dark hair, and was around the same height as Subaru. Before snapping his fingers, he closed his eyes and started to say with a confident tone of voice,
"Oh Natsuki Subaru, you will have quite the time on this journey. Let's just see how things unfold from here on out."
As the watcher said this, he snapped his fingers to see how Natsuki Subaru handles things.
Author's notes:
1. Subaru (from the main canon) will be heading into the worlds of the what ifs with an ally depending on the world itself for example the Lust if it will literally just be him since there is no need of having any allies there.
2. Subaru can't truly RBD but when his counterpart RBD, he also gets RBD just without the dying part. So if he dies and then What if Subaru dies, he RBD's as well. It's not until literally how the what if ends will Subaru be in a permanent dead end.
3. The what ifs I will focus on is Pride if, Wrath if, Sloth if, Greed if, Lust if (for mainly one chapter since it is a joke if), and Gluttony if (being the very last one)...maybe I can work on any others like the Al if story from the mobile game (assuming I know the plot itself) or other if stories from the fan community if you all want me to.
Honestly, I have wanted to do this after having gone through 5-6 arcs of my Kingdom Hearts x Re:Zero story (Kingdom Hearts: Saving life in Another world). So hopefully you all enjoy this story.