Tanjiros POV

I made my way up the mountain only to see a pale white untouched absolutely enormous Temple however before I could even walk near it, I was attacked! By a... Red sphere? Before I could even get a good look at it, the sphere fired a very powerful blast of red and dark pink crimson energy at me. Before anything else the creature moved sideways and then started floating it then took the appearance of a ginormous black skull as then a strange light green lime glow came from the mouth of the black skull and suddenly I felt much weaker and much more tired. However I wasn't going to let that take me out, so I grabbed my katana and began to perform the sun breathing technique or the hinokami kagura! However before I could make my first move a large pillar of crimson light surrounded him and pierced the heavens!

The next thing I know standing in the place of the black skull with a Red orb in the middle was a man with red skin and horns.

???: Stay down human, I wouldn't want to use giga drain on you again, after all it's a miracle you're not dead or in an eternal coma!

Tanjiro: what the heck is a giga drain?!

???: A move that drains the target of all their energy and gives it to the user of said move

Tanjiro: why...

???: Hmm?

Tanjiro: why am I alive, why are you not eating me?

???: Why would I do that human?

Tanjiro: because your a demon!

???: What is a demon?

Tanjiro: how do you not know!? You are one!

???: How dare you! I am nothing like these human eating beast you speak of!

Tanjiro: what are you then?!

Regidrago: I am the Regi, Regidrago!

Tanjiro: what's a Regi?

Regidrago: we were a race of all powerful beings! Each Regi was born with different powers and abilities as well as a title, my sisters had the power to create life and to extinguish it, to create the very ground you walk upon, to create the very metal your blades are made out of, one of my sisters even once created your Sun, however I was the most loved of our mother's children, our mother created everything with her hands alone, in fact if memory serves correctly she once hauled a entire planet around with one arm! However my mother created me to be an exception to my sisters, she made me male, but also she made it to where the core within my body, my very soul would never run out of energy as it is an infinite supply of energy. However my sister, Registeel believed that I would be the undoing of everything as I was too powerful and I had an infinite supply of Life Force, so with the help of my other sisters she stole some of our mothers devine power and used it to give life to appeared to be a solid ring of metal which made no sense and then a large amount of lightning pierced from the clouds, and then the lightning just like the ring, came to life it became Regileki who was made by my sisters to be my equal and the only thing capable of fighting me, however while she did not have an infinite amount of energy she did have one unique ability, after her body was out of energy and her life force was spent instead of turning to Stone and being put into an endless eternal slumber, her soul and consciousness remained intact, and so eventually when the time had come she would reform herself. However she has been missing since long ago when the great battle ended our race.

Woah this guy is some kind of God!?! Holy crap I should be careful.

Regidrago: anyways, could you tell me more about those demons you spoke of?

Tanjiro: of course!

Later

3rd POV

Tanjiro and Nezuko led the somewhat kind or friendly Regi to their home, the butterfly Mansion, however on their way back a certain yellow-eyed and pink-haired demon who was watching attacked.

Tanjiro: Regidrago come on!

Regidrago: it's okay I'll be fine don't worry about me, now go save your sister and yourself while I buy you some time.

Akaza: don't even think you'll stand a chance against me! I'm the fourth most powerful demon for a reason!

Regidrago:... Good now that we got the bystanders out of the way...

Suddenly Regidrago began to change drastically, at first it appeared he was just turning into his primal form but then a strange Aura started emanating from it

Akaza: this is your ultimate power?!? Pathetic!!!

Regidrago?: Dragon Energy!

Suddenly a blast of crimson energy would launch from Regidrago, the attack would have been so fast to the point where Akaza would not be able to dodge said attack, and not only was it fast but it was powerful too it disintegrated half of Akaza's body, the demon tried to regenerate but he found that it actually hurt him to regenerate now.

Akaza: Gaah! What have you done to me!?!?

Regidrago?: Don't complain about me harming you, you're the fourth strongest demon after all! Show me all of the power you can muster!

Akaza: fine then but it's your funeral! Blood demon art!: Destructive Death Compass Needle!

The demon then began launching powerful attacks of shock waves which came from just the air pressure of his fists moving, the blasts were so intense they even uprooted  trees! However despite his vast strength and immense speed, Akaza couldn't get a single hit on Regidrago?.

Akaza: GAAAARGGGGAAAAAARAAAAAAH!!!!!

Harnessing all of the rage and power he possibly could, Akaza launched 10 devastating attacks and each one hit Regidrago?!

Akaza: AHA! I FINALLY HIT YOU!!!

Regidrago?: Wow that actually hurt, welp because you attacked me with full power, I'm going to attack you with my full power!

Regidrago? Was then consumed by a very large crimson, dark reddish pink and black Aura which appeared to just stay still for a whole 10 seconds, and then it began to expand and grow it appeared even a black and red crimson tornado have begun spawning from on top of it! As we then see pieces of Shadow and darkness begin materializing into a physical shape as these physical pieces of darkness are then pulled into ginormous swirling vortex of crimson destruction. Suddenly The vortex grows 100 times it's size as then we hear a loud terrifying chuckle.

(0:47)

From the top of the vortex rises an absolutely large serpentine like beast as pieces of Shadow and darkness then combine with each other to create ginormous hands for this absolutely ginormous monstrosity.

(This art is not mine)

The ginormous beast then stairs down directly at Akaza

???: WHAT? ARE YOU SCARED?!?! HEHEHEHAHAHAHA!!!

Akaza: W-what... Are...y-you?!?!

???: YOU DARE ASK WHAT AM I? VERY WELL THEN..... I AM THE DARKNESS SURROUNDING THE WORLD, I AM VOID OF DESPAIR AND MISERY, I AM THE ONE WHO IS CALLED UPON AS THE BANE OF LIFE AND THE GREAT BEAST OF EXTINCTION, I.....AM.......
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REGIDRAGOMAX!!!!!