Warning: Aside of terrible grammar and poor vocabulary beyond comprehension, this is a story of OP Rimuru. At the end of web novel, Rimuru is indeed OP as hell. No one in DxD can force Rimuru to fight seriously, let alone going all out. No one can actually hurt him (his body is always protected by multiple dimensional barriers—not to mention Manas Ciel will ensure no harm can reach her master), let alone kill him. However, this is not a serious story (maybe). Don't worry about power level or anything.
—[The Strongest Being is a Slime!]—
A thousand years had passed since the end of Tenma War. The world had flourished: it had advanced to a level that his original world couldn't compete anymore. The new era had since long ago started.
But that didn't mean Rimuru was busy. On contrary, he had so muuuuch free time at hand. Well, he could alter the time to suit his taste. Free time was never a problem to begin with. Making a billion years into a second was not a problem at all. But Rimuru didn't do that. In spite of himself, Rimuru would like for everyone to have the same perception toward time.
His subordinates had taken care of everything for him. Guy had been given a planet for him to rule. So was Rudra—another reincarnation of him to be precise. Those two, and the others who had been given a planet to rule, would then give a report to Diablo once each a hundred years. Aside of acting as the overseer of the universe, Diablo himself also ruled Rimuru's former planet. In short, everyone was doing the jobs.
As a conclusion, Rimuru had nothing to do. He was alone in the throne of heaven—the realm of angel that was supposed to be ruled by Milim, but Milim was busy playing with her pet in an empty planet. One of his parallel existences was also joining her play, as she requested. Aside of Ciel, his dependable and beloved Manas, he had no one to converse with. Not that he felt lonely (he was basically everywhere and with anyone), but the boredom was real.
It was no wonder that Veldanava decided to live among his creations and married one of them!
"Master, don't try to follow that unacceptable path. No woman is good enough for you. I don't approve such thought."
Well, there his beloved Manas saying her usual things! However, when he asked the wisdom core about why there's no woman good enough for him? She became as silent as a stone, pretending she didn't listen to his question.
See? There's no response at all.
Anyway, even though Rimuru entertained the thought of marrying, he was actually a genderless being. Well, technically he could become a man or a woman at will, but it kind of against his nature. Naturally he did try to be a male and a woman a few times. But it was just that, merely to entertain himself.
Oh, don't think of anything perverted like him transforming into a woman to gaze at the "beauty" of her naked body! Rimuru didn't do that. Never. He was a gentleman. He didn't do such a thing. Well, perhaps there's one time he did that out of curiosity. And there's another time when he just wanted to see it. And there was that time…. Okay. He did like to do that. But so what? There's no wrong in admiring one's body!
"Master, that kind of interest…is not healthy."
"…Is…is there a feeling of wanted to be admired in that tone…?"
"…It is certainly your imagination."
Right! These days he had some kind of a feeling that Ciel acted like a girl who wanted her boyfriend to give his utmost attention for her. Actually, not just these days; it was every day. If Rimuru pondered deeply about it, Ciel started acting like that since she evolved from Ultimate Skill [Wisdom King Raphael] into Manas Ciel. Ever since, she always—
"There's no need to ponder about that, Master, it surely is your imagination. Besides, don't you want to do something to erase the boredom?"
Right! It was just his imagination! There's no way the all-knowing Ciel-sama was acting like a girl. What a troublesome imagination!
"Anyway, do you have a suggestion? Since you asked that question, surely you have a very good suggestion prepared."
That smugness is so easy to pin point! But, well, Rimuru couldn't say anything about Ciel's smugness. After all, Ciel was the all-knowing. There's nothing to be surprised about. If there's a problem that the Almighty and All-Knowing Ciel could not solve, then there's no one could solve it.
"Master, I'm All-Knowing. You're All-Mighty. Together we are unstoppable. But yes, I do have a very good suggestion."
"Well then, why don't you spill it?"
One of the things she did when she wasn't looking for a worthy individual to be included into her group was glaring murderously at the stupid-overgrown-idiot-lizard that had invaded her home. If by looking could erase someone from the very plain of existence, the self-proclaimed True Dragon had already ceased to exist for numerous times. But, no matter how much Ophis wished for it, the overgrown lizard was there hovering as it's please in the very home of hers.
Looking at its stupid form was enough to prompt the infinite dragon god to think of mutilating that filthy dragon alive. Sadly, despite her greatness, Ophis alone couldn't do that. Great Red was as strong as her, if not stronger. Should they fought at full power, the planet that allowed all life to exist would get destroyed. If that happened, even she would die. After all, just like Great Red that existed because of dream, she existed because of infinity. Nothingness was no infinity.
Despite how stupid that red lizard was, it was not going to destroy the planet. Naturally all supernatural beings wouldn't stand still should it want to do it. No matter how arrogant those weaklings were, they understood that human and the planet were the very two keys of their everlasting existence. And Ophis was way smarter than stupid red or those weaklings.
So, if she was to kill that overgrown lizard, she had to create a separate dimension strong enough for them to go all out. Unfortunately, despite the infinity she embodied, Ophis could not do that while fighting stupid red. That was one of the reason of why she created Khaos Brigade.
"Perhaps I should seal that thing rather than kill it, it would be easi—"
Ophis failed to finish her words as something graced the very dimensional gap. No, it didn't because she had felt it. She couldn't feel it with her power. The only reason she knew something graced it was because she was born in this very dimension. That thing…something had graced her home and passed it without actually entering it.
The planet…something was already on the surface of the planet. And…it lost. Just like that.
Ophis didn't wait for anything. She didn't know what it was, but she was sure that it could help her dealing with the stupid red. If it could enter the planet from the another side of the dimensional gap (which was actually connected to the border of the universe) without entering the dimensional gap, it should be strong.
Humming, Ophis entered the dimensional portal and left her home…temporarily.
"Downloading the universe' Akashic Record is finished. All information is acquired. Shall I make you the owner of the universe, Master?"
Wait! Wait! What about this universe's god? Will not he put up a fight? I don't want to look for a fight, you know?
"The center of the universe…is empty. The creator of this universe has yet to awaken. You can devour it and make this universe yours. There are many dimensional gaps surrounding many planets. If Master wants to do it without my assistance, Master just need to go to the center of this planet's dimensional gap and then Master can open the portal to the center of the universe. Well, of course Master can go there in no time, but we are talking about erasing Master's boredom."
…Right, Ciel tended to go overboard. He just wanted to amuse himself, not to conquer someone's universe. Indeed the owner had not yet gained consciousness, but still this was not his universe. He already had many universes to take care, why should he add another one? Why would he do something as troublesome as that?
"Well, let just enjoy this universe's earth…."