Author's Note: This crossover involves EoS Rimuru from the Tensura Web Novel (with some LN lore), as well as current Shiro and other characters in the Kumo Desu Web Novel (from right before the elf village raid). The story will mostly be from Rimuru's perspective (with some other perspectives).

This was something I wrote for fun in my freetime, so don't expect constant updates.

I started this story back in March, but didn't feel like releasing what I had until now.

A few things:

1. There is no romance or harem, and I'm keeping it that way (sorry, so don't suggest this)

2. I'm open to suggestions for future plot

3. Please don't ask for updates

4. Length of chapters may vary

5. I do not own any of the characters from That Time I Got Reincarnated as a Slime or So I'm a Spider, So What? The characters belong to their respective authors.

Text:

"abc" indicates verbal dialogue

"abc" indicates telepathic dialogue

abc indicates character's internal monologue / talking to oneself (or Ciel in Rimuru's case)

- indicates a short break

….. indicates silence

* * indicates a change in perspective, etc.

{abc} indicates Ciel talking

[abc] indicates _ talking (revealed later)

Chapter 1

*Rimuru's Perspective & Backstory*

It had been many millennia since the events of the Tenma War. After the war ended with Yuuki's defeat, Rimuru rebuilt his country of Tempest, and the Cardinal world prospered in peace. While there was the occasional mishap or other-dimensional vacation from Rimuru or his subordinates, things generally went smoothly. In fact, things had gone so smoothly that after a century of development, Tempest had fully developed the great Jura Forest, and most of the remainder of the continent.

The technology they had was a combination of magic and Earth technology, and the overall level of civilization had advanced far beyond that of Earth's. There were flying cars everywhere, with towering skyscrapers that were literally several miles high. The culture had progressed to the point that there wasn't much else that could have been done had they remained on the planet.

And so, a century after the Tenma War, Rimuru allowed for the development of space vehicles capable of transporting massive amounts of the population off the planet. The reason why he had these vehicles constructed, instead of simply teleporting the people himself, was because Rimuru had finally decided to step back from the day-to-day and year-to-year activities of his mortal subjects.

And so, spacecraft were developed using a fusion of magic and technology, and a mass migration of people thus occurred. They went to other planets in the universe capable of sustaining life, and settled there. Once the planet was fully colonized and developed, they would then create more spacecraft and look to even more planets. This cycle of migration, civilization development, and migration again eventually led to the former country of Tempest, now known as the Tempest Galactic Federation, fully colonizing and developing several galaxies.

They had become a Type 3 civilization on the Kardashev scale.

At this point, many, many millennia had passed since the Tenma War, and all of Rimuru's old mortal friends had long passed away. While most of his immortal subordinates remained, his human friends had long ago reached the end of their lifespans. Rimuru had offered to make his mortal friends immortal, as such would be an easy feat for him, but was declined by all of them in the end.

"All of us are honored to have been your friend, Rimuru. But in the end, we can't accept this gift. It is time for us to pass on", someone had said. Rimuru couldn't remember who.

When this happened, Rimuru was sad with regret, but had long ago prepared himself for this day. On this day, Rimuru ordered all of his subordinates to withdraw from the mortal realm and cease any further involvement with those with finite lifespans.

Thus, the founders of the Tempest Federation finally withdrew from the mortal realm. Humanity, demi-humans, and monsters were left to prosper without any further guidance from their gods. For the next several million years, not much happened. Rimuru and his subordinates spent their time lounging around, occasionally visiting the mortal realm and having some fun. Mostly; however, they simply lived a life of boredom. There wasn't much to do besides sit by and watch the mortal races for fun.

However, this was not the end of Rimuru's tale. During this time, he had sent one of his parallel existences to the end of space and time, and had it collect an infinite amount of energy using[Nihility Collapse] of [Void God Azathoth]. This caused his skill, which was originally inanimate to fully awaken into Manas: Azathoth. With this, Rimuru surpassed omnipotence and became the center of all of creation.

Interesting side note: According to Ciel, the skill [Void God Azathoth] was originally just a seed. Thanks to the watering of that seed from infinite energy, it successfully evolved into Manas: Azathoth.

(A/N: Yes, Manas: Azathoth is the same Azathoth from Lovecraftian lore. A.k.a. Rimuru is now a Tier 0: Boundless being on VS Battles)

Due to the supremely omnipotent power of Azathoth, the Devourer of All; however, several problems emerged. When Azathoth first awoke, all of existence immediately ended and was reabsorbed into Azathoth. This was quite a problem, as it left Rimuru with nothing to do for all eternity. To deal with this, Rimuru had to put Azathoth, his new Manas, back to sleep, in order for existence to continue once more. This did not mean that Azathoth was now completely dormant, but rather that most of Azathoth's power and consciousness was sealed, with a tiny sliver left awake so that Rimuru had someone to converse with occasionally.

Next, to deal with the issue of the end of existence, Rimuru had to recreate countless outerverses down to the individual atom. Rimuru did not interfere with his creation, and instead let it develop on its own. In one particular multiverse, eventually the planet called Earth once again was born, and humanity once again sprouted forth. Unlike the original; however, this one did not have the planet of the Cardinal world.

Additionally, Rimuru's subordinates were missing from the picture. When Azathoth awakened, all of existence besides Rimuru had been absorbed, and this included everyone Rimuru loved and treated as family. The pain and feeling of loss stemming from the fact that the whole thing was Rimuru's fault drove Rimuru into a depression lasting billions of years. Just like during the Tenma War, Rimuru refused to recreate his family, as they would simply just be his creations. Rimuru could never treat them the same way as before, so he resolved himself to being alone for all eternity.

The reason why he couldn't simply travel to the past again, was because all of existence had already ended. Even if Rimuru went to the past, the past would already have ended with existence, since Azathoth consumed it all. The only way for Rimuru to see his family again would be to recreate them anew, something that was unthinkable for Rimuru.

In their place, a whole host of other semi-spiritual deities, gods, titans, and other beings developed from the magicules lying around in the multiverse. The strength of each of these beings varied greatly. However, generally most of these beings called themselves "gods" and ranked themselves based on their respective strengths. The weakest gods could hardly call themselves gods at all. In fact, compared to the strength of the Cardinal world, they would only be ranked Special A or so. The only reason why they were "gods" in the first place was because they had infinite lifespans, and thus weren't mortal.

The middle-strength gods were ranked S or so, while the upper tier gods had a strength of Special S. In the end; however, none of them had the strength to compare to Rimuru, considering the entire multiverse was created by Rimuru himself.

Nevertheless, it was into this new multiverse that Rimuru stepped foot into, in order to have more fun and enjoyment. The first thing he did was create a few planets, onto which he planted books containing hints as a sort of treasure hunt. The "treasure" these books led to was none other than a scroll containing the truth of the world, or at least that was the way it was phrased. In all honesty, it was just a copy of Rimuru's diary.

Next, he found a deserted planet and settled down for a short little nap. In mortal years, this short nap of his was somewhere around 4.6 billion years, but to Rimuru who had already lived for an eternity, this was nothing. Rimuru also directed Ciel to wake him either when the 4.6 billion years had passed, or when there was something interesting which could amuse him.

And so, Rimuru Tempest settled into his short little nap.

4.6 billion years later

Rise and shine! I thought to myself as I woke up.

{You have slept for the entire duration of 4.6 billion years.}

4.6 billion years, huh? That felt like a second to me though. Ah well. That's not important right now. Instead, what IS important is:

WHY AM I COMPLETELY BURIED IN LAVA?! I thought I had even specifically built a structure to house my hibernating body in. Did it get destroyed or something?

{Master is currently located 1,000 meters below the surface of this planet.}

Yup. I got buried alright. How did this even happen? I thought I even used some decently advanced magic to secure my structure. It wouldn't have been destroyed by some natural cause such as a meteor, even. Oh well, it's not like anyone could have damaged my body anyways, so we're all good. For now though, it would be boring if I simply teleported out of here, so let's see if I can make my way out.

Let's begin with using Universal Detect, shall we?

After turning it on and limiting the range to a few miles instead of the entire planet, the first thing I noticed was the enormously large cavern that this lava lake was in. For miles around, the only thing I could detect was endless amounts of lava. Hm? Wait a second.. I take that back. Looks like I've found something interesting! There appears to be some sort of large catfish swimming around in the lava nearby. Does it have heat resistance? In fact, this sort of creature must've evolved somehow or another in the past 4.6 billion years right?

Interesting. I thought to myself. Out of all the possible planets, somehow this one is capable of sustaining life. Perhaps I should go and see if I can discover other forms of life on this planet.

And so, I transformed into my humanoid form resembling a 20 year old, put on my favorite set of clothes: my black Demon Lord Kimono, and proceeded to fly out of the lava and into the air gap between the surface and the ceiling. In the distance, I saw the catfish-like creature in the lava. Once I got close to it, it appeared to be roughly 20 feet long, and was covered in some sort of scale.

Hmm. Catfish eh? I'm in the mood for some seafood.

And so, I proceeded to cast a little bit of black lightning on the catfish, which instantly killed it as well as gave the raw meat underneath a good searing. I tried out a bit of the fish, and to my surprise, it tasted quite realistic to seafood on Earth!

After my meal, I observed my surroundings again, and noticed the large cavern that I was in wasn't a sealed off cavern at all, but instead had a sort of labyrinth-like feel to it. In fact, I noticed several passageways in the distance which appeared to be leading towards the surface.

I then flew towards these passageways and proceeded to walk up the passage. After walking for around 5 minutes or so, I arrived in another sort of cavern, although this time there were several peculiarities lying around. First of all, I noticed there were invisible strings of spider webs hanging all over the place. 2nd, there were several hundred white spiders running around in the cavern.

Since the webs were in my way, I just brushed them aside and kept walking along the path, trying to avoid the spiders that were crawling around. Strangely enough though, when I brushed aside the webs, all of the spiders immediately stopped moving and started staring at me. In fact, I could even detect some killing aura coming from their eyes.

Whelp, I better get out of here quickly then! Looks like I'm not invited into their den.

So I decided to just teleport up to the surface of the planet.

Directly below my feet was a sparkling ocean, and there was no land around in sight for miles.

Huh. I guess sometime in the last 4.6 billion years water eventually formed on this planet? It seems to be doing quite well, in fact.

No sooner had I entertained this thought before:

{I believe it would be a good idea for Master to first observe the opposite side of the planet.}

Other side? Let's check it out with Universal Detect.

"..."

HEY HEY HEY! OI! WHAT'S GOING ON HERE?! WHY IS HALF OF THE PLANET DESTROYED?!

From Universal Detect, I noticed that the other side of the planet was covered in thousand-mile deep fractures and canyons. There was no life on the other side; only the presence of rock, dirt, and lava.

Ciel, what's going on here?

{Would you like me to spoil you? Or would you like to find out yourself, Master?}

Grr. Looks like I'll have to uncover the truth myself then. After all, when I first created this planet, there weren't any lava or such fractures. In fact, it was just a plain 'ol ball of dirt.

Actually… Now that I think about it.. Isn't the density of magicules here much lower compared to before? Could that have anything to do with it?

{...}

Looks like I'm on the right track. For now though, why don't I go head towards land? Since there appears to be catfish and signs of life here, does that mean there are also humans?

{Correct.}

Great! Hmm.. normally in situations like these I would prefer going by boat and leisurely enjoying my time, but it seems like there are water dragons in the ocean. It's probably best if I just fly over. Let's hope they don't glare at me like those spiders did earlier.

Using [Gravity Manipulation] I then accelerated myself up to Mach 1 and enjoyed a short 10 minute scenic flight over to the nearest landmass. Upon reaching the shore, I slowed myself down and landed on the soft sandy beach. Next to the beach was a dense, lush forest. Further in, I could sense a small town next to a river.

Since it looked like there were humans in the town, I decided to head on over and check out the culture in this world.

After walking for 5 minutes or so, I arrived at the town gates. Flanked on either side appeared to be soldiers, and they had some sort of crest on their armors. When I walked up to them, they seemed to at first be doubting their eyes. There was no doubt that the reason for their abnormal behavior was due to my humanoid body's looks, as well as the unnatural-looking jet-black clothing I was wearing. Ah well, it would be a bother to try to explain myself, so let's just hypnotize them to let me pass.

*Waves hand at soldiers for dramatic effect*

I then casually strolled inside after successfully hypnotizing them. I'm not trespassing, alright? Since I'm the one who created the planet, after all.

The first thing I looked for was a restaurant. Why? Because I love food and haven't eaten in 4.6 billion years. Not that I needed to eat anyways. And so, I headed into the nearest restaurant to go eat some good food.

Meanwhile, in a certain Demon Lord's Castle in the Demon Territory

*Shiraori's Perspective*

Eh?! What's this? My babies which I tossed into the Great Elroe Labyrinth just reported something shocking to me.

Childcare? What's that?

They're spiders, not humans. They don't get childcare rights.

Anyways, I was busy enjoying tea time with the Demon Lord, when out of nowhere, some of my clones which were mixed with my babies picked up on the most peculiar thing.

Like seriously! I'm SUPER busy! I have a war going on, ok?! I don't have time for this!

Annoyed, I took a look at what my clones were reporting, and boy did I almost spit the tea right back out of my mouth.

There, right in front of hundreds of my clones, a single girl (or woman?) was casually walking.

As I watched, they brought one hand down in front of their path, and in one fell swoop cleared out thousands of tiny spider webs.

HEY HEY! THAT'S MY HOME, YOU KNOW!

YOU CAN'T JUST GO WALTZING INTO SOMEONE'S HOME ACTING LIKE THAT!

By now, I was super mad. It's not like I have trauma over losing my home, ok?

Yup, totally not traumatized.

The woman also had the weirdest fashion sense ever. Their outfit was almost entirely black, except for a small gold-colored strip vertically down the middle of their cloak.

What? My fashion choice is perfectly acceptable. I'm naturally white, thank you very much. No other clothing goes well with my albino skin color.

I'm not like a certain administrator that I know who only wears black…

Wait… There's no way, right?

ARGH whatever!

Thinking about it won't help.

Turning my attention back towards the mysterious woman, I watched as she kept walking along, bulldozing the houses of my precious babies.

THAT'S THE FINAL STRAW!

Get ready to face despair! Evil Eyes of Extinction, activate! What?! It's definitely not overkill for this, ok?! This person trampled all over the home of my babies. They should at least deserve a painless death, right?

*No effect*

Huh. No effect.

Say that again?

No effect?!

I said, Evil Eyes of Extinction, activate!

*No effect*

WHAAATTT?!

Now, I was immediately on high alert. In fact, I was so startled that I accidentally stood up and knocked over my precious tea cup. The Demon Lord glanced at me; she was definitely thinking something was wrong.

"White, you ok?"

No, I'm definitely not ok, thank you very much! This unknown woman just waltzed into the home of my babies in the Great Elroe Labyrinth, took down all of the spider webs with her bare hands, and then proceeded to survive my Evil Eyes of Extinction?!

In fact, what's she even doing in the Great Elroe Labyrinth, anyways? How did she even get in there?

While I was thinking this to myself, the mysterious woman suddenly disappeared.

WHAT?!

Just that.. Was that teleportation? Even more, how did I, a master at teleportation, not even detect a single thing? The only thing that could compare to that level of teleportation would be D. But didn't she say that she was just an observer? And this person looked nothing like D.

On closer thought, that person's face was even more beautiful than D's!

How in the world is that possible? I thought D made herself the most beautiful person in the world just because she could.

Ok, looks like this is much more serious than I had thought.

Message to all clones. Search for the mysterious woman that was encountered in the Great Elroe Labyrinth immediately. Top-priority.

Ok, hopefully they'll let me know the moment something comes up again.

10 minutes later

What's this? One of my clones in the Ohts Empire just reported to me that the mysterious person just walked right into the capital?! Like.. Straight up.. Walked in? I thought they had some trained guards around that place carefully controlling traffic.

Wait, hold on. Apparently they also waved their hand or something? Is that some kind of hypnosis where they're like, "I am not the suspicious person you are looking for" thing?

Anyways, they seem to have entered a restaurant now, so let's observe in the meantime.

Hey you. Yeah the clone that's lazily sitting on the rooftop over there. Get your butt over to that restaurant window and report to me on what you see.

Hmm. Hopefully things turn out ok, but I've got a bad feeling about this….