Return to the Past for the True Happy Ending.

Prologue

Talking: "hello"
Thought (hello)

1 year after Rimuru defeated Yuki.

Rimuru is going his regular routine, he starts off by checking in with Korube.
He then checks the daily results of the labyrinth, then he watches Shuna, Shion, and Milim fight over him.
Rimuru then checks out the goblin riders' daily training against Hakuro.

Rimuru: Today is so peaceful, it has been about a year by now since I defeated Yuki.
Gobta: Peaceful for you, I have to try to not get beaten up by the old man again...n...n...
Hakuro while looking annoyed at Gobta: looks like this lesson is gonna last longer again.
Gobta while terrified: I ment to say sensei!!!!

Rimuru chuckles a bit.
Hakuro: We all can live happily like this all thanks to you lord Rimuru, with your strength nothing can go horribly wrong.
Rimuru (Usually saying that would be a bad omen, but because of me nothing will actually happen, because nobody is stupid enough to attack us)

Soei: "Lord Rimuru, we have spotted a person moving at very high speeds into the forest, it is not anybody we know, he would be arriving at your location in a few minutes."

Rimuru (of course a person is after me, welp it is nothing that I can't handle)

Then out of the forest appears a person dressed in pitch black armor who wields a blade that is as dark as space itself.

Rimuru: "Who are you and why do you want to fight me."
Armored person: "You are the only being that is in my way of erasing this universe that Veldenava created".
Rimuru: (This guy is bad news, not only he got here so fast but he seems confident that he can accomplish his goal, ciel what skills does he have)
Ciel while scared says: "that is impossible I can't analyze anything about him, he must not be part of system"
Rimuru: (even if he is not apart of the system, we have survived at the end of time and space itself, so we have the power to defeat him)

Then suddenly the armored person attacks with godlike speed and slices off one of Rimuru's arms, Rimuru then jumps back.
Rimuru: "You are impressive to be able to cut off one of my legs limbs, but you'll be disappointed because even if I have 1 cell left I can fully regenerate instantly."

Rimuru then attempts to regenerate his arm, but nothing happens, Rimuru is confused of why his arm did not regenerate .
Ciel while scared: "impossible he is able to stop the regeneration that has my authority".
Rimuru then gets nervous and brings out his sword, "The Blade of Rimuru Tempest", this sword is a product of Ciel combining: "The Sword of Tempest" with "Veldenava Sword" and was made to absolute perfection.

The person swing his sword at Rimuru again Rimuru goes to block the attack with his own sword, but as the swords clashed, Rimuru's sword snapped with one hit, and the attack took Rimuru's other arm.

Hakuro, Gobta, and Soei then rush in to protect Rimuru.
Soei: "Lord Rimuru you must flee, we will guard you until you regain your limbs."
Rimuru aims one of his arms at the Armored assailant and combines his powers to launch spell he modified to deal massive damage.
Rimuru: "Infinite Atomic Devestation!!!"
Infinite Atomic Devestation is the result of Rimuru using the probability alteration of Veldora's ultimate skill and the partial acceleration of Velgrend's ultimate skill to enhance nuclear cannon into the most powerful ranged attack and because of "turn null" and "infinite space", he can fire off the spell as much as he wants.
The barrage hits him head on, the barrage lasted 10 seconds, and when the smoke cleared.
Rimuru becomes terrified as he sees that as the dust clears, the assailant is still there a not a scratch on him.

Assailant: "Is that all, I guess this is what I'd expect from a person who has regrets."

Gobta and Hakuro charges at the assailant.
Hakuro: "Lord Rimuru run, we can't win in this situation, as long as you live we can come back and win another day."
Rimuru: "I can't leave you guys behind, even if I can resurrect you, I would have to live with the guilt."
Soei: "Lord Rimuru, I am sorry but this is the only way, both of your arms can't regenerate, your sword has been destroyed, and he is not affected but the barrage that you just did, as long as you live you figure out how to beat him."
Rimuru while upset teleports to his capital alone and activated the emergency system to bring the whole capital into the labyrinth.
Rimuru using voice communication to all of his subordinates: "Everyone we have a emergency, there is a powerful foe coming for me that I can't beat, my regeneration is disabled and my sword has been destroyed, I need time to analyze the attacker and gain back my regeneration."
Then all of Rimuru's subordinates teleport to him including Veldora. Rimuru also sent out a message to all the other Octogram members and to Velgrynd for help.
Then suddenly the assailant busts into the labyrinth directly to where Rimuru and the others are.
Kurama immediately brings out all her summons to put as much rebuffs on the assailant while Adalman along with Venti and Albert barrage the assailant as they can, but all 4 of them were instantly wiped out when the smoked cleared.
The assailant rushed towards Rimuru, but Geld intercepted them and took the hit instead causing Geld to also be killed, Gabiru then the quickly charged at them, the assailant tries to counter him, but Gabiru used his ultimate skill to dodge the attack and land a hit that only deflected off of him, which allowed the assailant to take out Gabiru.
Shion and Ranga charge in next
Shion: "Ranga let's do the church maneuver"
Ranga uses his skills to enhance Shion's blade with lighting so it increases the chance of dealing damage so she can activate her unique skill master chef to cause the smallest amount of damage to instantly kill a target.
She lands the hit but it did not do damage and just got deflected, the assailant then took Ranga and Shion's lives.
The demoness trio then launch a barrage of attacks at them with all their strength, but they were killed instead.
As the demoness trio died, the rest of Octagram showed up along with Velgrynd and Velzard.
Ciel: "I think I have found a piece of information, it is resonating with Akashic Records, there may be a hidden skill within it."
Guy Crimson: "what is going on here, why are you being overpowered like this?"
Rimuru: "Be careful everyone this guys has a Ultimate skill that I don't have which is able to overpower me."
Dino: "how are we suppose to beat him then."
Rimuru: "because I now know that it is an ultimate skill that caused it, I can find out which one it is and how to counter it."
Dino, Carrion, Fey, Valatine, and Dragule all charge at the assailant to buy time.
Rimuru: (hurry up Ciel, they can't hold off this guy forever.)
As the the last of the ones who attacked the assailant is left Ciel found the skill.
Ciel: "I found it, it is called, Origin skill Regret Ruler, this skill will make make the weirder unaffected by the attacks and defense of anyone who has regret."
Rimuru: "how are we suppose to win then?"
Ciel: "the only way we can beat him is for you to return to the past so you can fix your regrets, Time-Leap needs time to charge because of the hits he landed earlier."

Benimaru then fires off a hell flare but got killed by the him.
Zegion attacks with full strength but only to have his exoskeleton shattered.
Ramiris tries to use the labyrinth to resurrect everyone but she was instantly killed.
Diablo, guy crimson, and Leon then attack him but were killed.
Ciel: "time leap is 80% complete."
Velodora, Velgrynd, and velzard transform into their dragon form while milim transforms into her combat form and they all launch their strongest attack with all of their being, but the assailant didn't even flinch and instantly took them out.

Assailant: "You have no more pawns left, now be a good prey and let me kill you."
Rimuru: "they are not my pawns they are my friends!"
The assailant agitated swings his sword at Rimuru.
Rimuru closes his eyes prepared to be killed but he didn't feel a thing as he hear a familiar voice scream, the one he wouldn't ever expect.
Not wanting to believe it to be true he opened his eyes and sees a clone of himself taking the hit, The clone head turns to face him and he sees red eyes instead of his normal ones.
Rimuru: "ciel, why would you sacrifice yourself?"
Ciel: "this is what had to be done so that TimeLeap would be able to activate, if there was another way I wouldn't have sacrificed myself. I will use all of our strength to hold him off until time leap is ready, this is risky because you will most likely loose a great portion of your strength and skills."
Rimuru: "you don't have to sacrifice yourself when time leap is ready."
Ciel: "Don't you get it, he can interfere with our skills which mean he can either follow you when you activate time leap or just negate the spell altogether."
Ciel then uses every singe skill she has at her disposal to hold back the assailant.
As the assailant is getting close to her, Ciel finishes activating time leap.
Ciel: "activate time leap."
She then uses her power to force Rimuru to be affected by it and as soon as the spell finishes.
The assailant grins and takes out Ciel.
Rimuru then passes out as he travels through time into the past.