My math is probably wrong, but,

5 months just passed in Assiah
So 41.6 years passed in Gahenna

Cuz 1 year in Assiah = 100 years in Gahenna
So 365 days A = 36,500 days G

Almost 42 years (but not quite ) have just passed in Gahenna, Rin is chronologically 57 years old, but demons stop ageing after 18- 30 years of biological age (in this fan fic). So Rin will have stopped aging completely by now, so I'm gonna say Rin is 19.

Bio age : 19 (physically and mentally)
Chrono age: 57 (time wise)

I like to think of it as how Aang is biologically 12, but chronologically he's 112 cuz he spent 100 years frozen in ice.

Think of it like that

I like to make my fan fics accurate, so I took the time to do the math.

Now I'm just rambling, you don't gotta read all this.

Actually it's kinda important that you do

Now enjoy the chapter.

Sorry for the wait.

**Cringe warning!!**

*~*~*~*

(3rd person p.o.v)

Every single demon had returned to Gehenna for the day, for today a truly momentous occasion was taking place. 

For the first time in over 10,000 years, a new demon king would be crowned.

Years of preparation had been put into making this day absolutely perfect.

Everything was perfect, the castle had been decorated with blue fire lanterns, and there was not a single red cloud in the purple sky above.

Today was Rin's day, years of hard work, training, and dedication will now all be payed off.
He spent 42 years learning all the ways and customs of Gahenna, learning how to lead a nation in the even that something were to happen to the demons supreme king. He had spent 42 years physically training, learning how to properly fight in a battle and used all of the available weapons. He now knew how to fully control and manipulate his flames. He could do all sorts of tricks with them. How to draw his sword without a single part of his body sprouting blue fire. He had complete control of when his flames appeared no matter what his emotion which took years of mastering how to control his feelings and manage his temper. He knew how to make his wings of fire appear which actually worked and could make soar high above the ground. He could summon any weapon he wanted at any time if needed. He learned how to speak all demon tongues fluently as well.

He definitely was not the same kid he was years ago. He was stronger, more mature, and much more calm. He even learned to keep his tail tucked away, no matter how uncomfortable the constriction felt.

Finally, his time had come.
Today, he was going to get his new title as demon king, new powers and forms unlocked, and his very own small kingdom to rule over.

Currently Rin stood in his room preparing for his coronation. The maids and servants helped to dress him to make sure he looked like the powerful leader he will soon be.
He stood extra still while a maid with pink hair fitted his suit. Amaimon floated near by watching as they prepared Rin for his coronation.

"So Rin, are you excited to become a demon king?" Amaimon asked in his usual monotone voice.

"Of course I'm excited." He said, lifting his arms as the ladies altered and tightened his clothing.

"You don't seem very excited." Amaimon said resting his chin on his palm sitting cross legged in mid air.

"You're one to talk." Voice laced with sarcasm. "Were you excited when you were crowned demon king?"

"I guess. But I don't know why, nothing interesting ever happens. There's never anything to do unless father or our brothers ask me to do something for them. Are you almost done yet?"

"Have patience onii-San, you're acting more restless and antsy than me, and I'm the one being coronated." 

Amaimon let out a small, almost inaudible huff looking away. Most people can't tell what Amaimon is feeling, only those who pay close attention to him could tell. Rin let out a chuckle. "Almost done." He said with a small smile.

"The coronation is almost starting, you're going to be late for your own coronation."

"As you just said, it's my coronation, it's not going to start with out me. Without the one getting coronated there is no coronation. It will start when I get there."

"There you go my lord, all finished." The maid said bowing to get future king and exiting the room to let him be and prepare. All three ladies left without another word.

"Finally, let's go." Amaimon said grabbing Rin's hand leading him out of his bedroom, something he only did to Rin and him alone.

Amaimon was tugged back when Rin didn't follow him. "What's the hold up?" He asked looking back at Rin. He noticed him staring at the floor playing with his fingers nervously. Something he hadn't seen him do since he was much much younger.
"What's with you?" He asked again.

"Umm. What if I can't do this?" He asked, still looking down.

"Can't do what?" He asked, letting go of his hand.

"This. What if I'm not a good king? What if I'm not fit to lead an entire kingdom. What if I run it into the ground and it all crashes and burns? I'll go down in history as the worst demon king to ever come into power."

"...."

"..."

"Now we all know that's not true." Amaimon said. "Father chose you to be a king, and father doesn't make mistakes." Rin looked up from the ground. "You're the only one with blue flames, that makes you technically the most powerful kin of Satan, I guess. You're also probably the kindest one in all of Gahenna,which I used to think was a weakness, but I guess you changed my mind some how. You're probably the most qualified out of all of us to rule, next to father that is. You're not going to be a horrible king, you're just doubting yourself because you're nervous. Demon kings don't second guess themselves, so you'll have to learn to cut that out."

"That...is probably the nicest thing I've ever heard you say." Rin said in shock.

"Don't get used to it." He said which caused Rin to let out a small chuckled.

Suddenly Rin launched himself at Amaimon, wrapping his arms around him in a hug, taking him completely by surprise. "Thank you." He said softly. "I think I needed to hear that." Amaimon stood there frozen. "You're not so bad, actually."

"...I don't think you're too bad either." He awkwardly pat Rin on the back, since hugs and physical contact wasn't really his thing. "So, are you ready to go know?"

"Yeah, I think so." He nodded, pulling away from the hug.

~*~*~*~*

A demon king crowning was such a special occasion, that literally every demon that existed was gathered in Gehenna. Some were lucky enough to be able gather at Satan's palace to see it in person. Others who weren't so fortunate needed watch or listen from their own living spaces. No demons resided in Assiah, which confused the exorcists to no end. Suddenly every demon just left. In the midsts of battle, the demons seemed to just scurry away from the battle. Leaving the exorcists to wonder if they had won, or if some greater threat scared them off.

Currently in Gehenna, at Satan's palace of darkness which stood in the centre of Gehenna's capital.
Up near the front row stood all of the the demon kings, it was rare to see all of them in the same place all at once. Usually they where all in their own kingdom, or out completing a mission they were assigned to from Satan. Or if they were like Mephisto and Amaimon and sided with the human's and exorcists. Well, Amaimon didn't necessarily side with with the humans, but he was basically like Mephisto's pet and always did whatever he made him do.
Satan was not too happy to see his three sons Samael, Amaimon and Lucifer. He definitely was not pleased that they had chosen to work with humans, Satan's enemy. But he decided to let their presence slide since more important matters would be taking place today. Satan was waiting for the day he would crown his youngest and most powerful child a demon king. Who knows of his true potential now that his demonic side will fully be unlocked.
All the demon Kings were dressed in their best attire, they needed to look presentable for this occasion.

Both Satan and his Queen Yuri sat at their throne. He wore his best royal robe and cape , his wings of fire were huge and blazing brightly. His eyes were bright blue with glowing red pupils with a look of pure determination. He was more exited for this day than his own son being crowned. Yuri wore her best Gehennan dress and a shining black crown. She was absolutely stunning. She too was excited but not for the same reason as Satan. She hardly ever saw her own son which was very sad. It seemed as though they were both busy doing their own thing. As the queen she was always busy and almost always by her husbands side. And as a demon king in training Rin was always busy...well...training. But today she will see her son, and on one of the most important days of his eternal life. She missed her son's dearly. She loved the two of them with all her heart, despite all of the horrible choices her youngest child was currently making. She pitied him and hoped that one day he would see the wrong of his actions. He hurt his only brother and twin no less, and by hurting his only living family, he hurt himself. It wounded her both emotionally and physically to see her first child in so much pain, she was glad that her husband was able to bring him home like she had requested long ago. She wanted him safe and happy and now he was. A mother always looks after her child.

She watched as Amaimon, her step child, hurried over and took a seat with the rest of his brothers, not long after the sound of instruments filled the palaces grand hall.

"This is it my love." Satan said with a grin that could be seen as twisted to an outsider, but she could tell that he was just excited. His hand found its way around his wife's own.

The large entrance doors were pulled open by the servants to the sound of an instrument similar to a trumpet. As the door fully opened, there she saw her son standing straight up with perfect posture and proud just as he was trained to. He wore a blank expression as he stood there. He was fully grown and looked much more mature than when she last saw him many moons ago. She was proud of him, look at how much he had grown. He started making his way down the aisle to the front of the room.

(A/N: guys, idk how this works so I'm gonna make it up)

Once he made it to the front of the room near the thrones he stood as Satan rose from his seat. He turned towards the audience of demons and living dead as they cheered, the crowds seemingly endless, the gateway of the palace was left open for viewers outside the watch the celebration. Every important being in hell was gathered in the palace. All of the demons kings and rulers of hell, Dukes and Duchesses of the realms of Hell, highest ranked beasts and demons, and the servants who had to stand waiting for a command at any moment.

"Silence." Satan said instantly quieting down the massive crowds. He had been waiting for this moment for many years and wanted to hurry this along. "We are all gathered here today to witness the crowning of the 9th king of Gehenna." He paused as some in the crowd cheered and howled. "This day shall be marked as the beginning of a new era. My long lost son, and your prince has finally returned to the place he rightfully belongs. It has certainly been a rough couple of years with the rise of exorcists and their growing technology and knowledge of our way of life. But your brief time of suffering has now comes to an end. Because now your new ruler had returned, he will be a powerful and fierce ruler because he has inherited the blue flame." This caused the crowds to erupt, the blue flames was very powerful and unique and no other ruler, let alone demon was able to fully weald it but Satan himself. "With him now on our side, humans and exorcists will not stand a chance over us, and the rein of the Grigori will come to an eternal end!" The crowds cheered even louder. "The exorcists have wronged us all, now we shall get our much awaited revenge." He turned to Rin. "Do you, Rin, solemnly swear to follow the laws of this realm?"

"I swear." He said sounding as confident as he could given the weight and importance of this situation.

"Do you pledge all loyalty to Gahenna and the thrown?"

"I do."

"And do you promise to rule over the subjects of your kingdom, and over all citizens of the realm in the event that the current supreme ruler shall fall?"

"I promise."

"And do you realize that there is no turning back after this." He said more quietly to Rin alone.

"I know." He said softly.

"You're doing great son." He said with the smallest smile.

"Thanks, dad." They shared a special father son look before Satan cleared his throat and continued.

"We begin the transformation." He said, his voice booming. The crowed cheered, this was the moment most have came for, the moment the prince is transformed into a demon king.

Rin kneeled before Satan, closing his eyes, as had been practiced during his coronation preparation training. Satan placed his hand over Rin's head beginning to say ancient chant. His hand glowed, covered in bright blue flames as he spoke. Slowly the transformation began. Rin's entire body emitted a blue light as his features slowly began to change. His hair became a brighter shade of blue rather than his normal blue-ish black. His ears became a tad bit more elongated and pointy, and his skin paled into almost a sickly whiter colour. The crowds watched in awe as their youngest prince transformed into something new, something much more powerful.

As the demon God finished his chant, Rin's glow had gradually dimmed. One the light had completely died down he stood once again and faced the crowd. When he opened his eyes, they were like new. They were a much more bright and vibrant sapphire colour, with crimson red pupils with a darker blue lining the iris.

"Your new ruler, The demon king of darkness." The whole crowd howled and cheered. This would mark an important day in history, the day that a new king would be crowned. The beginning of a new era, and the beginning of the exorcists downfall.

~*~*~*~*~

How does this have 5k reads??
This story is pretty terrible compared to others 🤔

How demon kings are made, I do no know. I just just made it in this fic that they were chosen by Satan. Sorry if it ain't accurate.

I really gotta catch up on the manga, I watched every episode of blue exorcist and the movie, but not all of the manga which I guess leaves me at a disadvantage. So far I'm just basing this off of whatever knowledge I have and much research. And my OC versions of the demon kings since I've never seen them all.

This is unedited. Sorry for the typos.