Demon Lord's Reincarnation

After defeating the heroes, Ren was able to easily march into the allied kingdoms' base. The leaders of the allied kingdoms didn't even dare to speak when face to face with Ren much less showed any resistance when he gave his usual ultimatum. 

"Serve me or die." 

At first, the members of the allied kingdoms planned to fight Ren to the death. They weren't scared of him even after hearing all of his legends. The stories had made him seem more than a single man, he was like some sort of God of War that no one could best. They all felt that the stories were grossly exaggerated, that was until today. 

Now that they have seen Ren they had a completely different view. The stories, the legends about him were not enough to describe the horror that he represented to his enemies. He was once likened to a God of War, but those that looked at him today were convinced that was an understatement, as they felt that Ren was the very embodiment of battle itself. 

All of the people present chose to submit to Ren. They had no choice since the moment they saw Ren, they knew that even if they fought to the end, they would be annihilated without achieving anything. 

The victory was anticlimatic but Ren still felt somewhat satisfied after battling the heroes and his companions. 

...

After Ren took over the allied kingdoms, he had practically taken over all the races and kingdoms within the continent. There was nothing left to conquer on land, once he was done Ren started developing his empire. 

The first thing he did once he was the absolute ruler was to fix the roads and create new ones that connect everywhere.

With the help of the transmigrators who had shared their knowledge of the technology in their own world, such as trains, Ren used these transmigrators' ideas alongside the technology within the holy kingdom to create a railway system. Since he had the greatest alchemist (Lara) and the best craftsmen (dwarves) and engineers (humans) on his side the railway system was established quite nicely. 

Now what once took months of travel takes around a few hours. The next thing Ren did was create schools for not only noble children but for everyone, no matter the race or social standing every child was to attend school.

In this school, all children were taught basic reading, writing, and arithmetic. Once they graduate from school depending on their talent they would be placed in the appropriate place. 

An example of that would be if one had the talent with magic, he/she would be placed in the magicians' tower to further their studies. Those with the talent of swordsmanship will head to the academy for knights. The ones talented in blacksmithing will be taught by the dwarves themselves, and so and so forth. 

In the span of a few years, many geniuses rose to fame and Ren's Empire grew powerful by the day. Even his brother had grown into a powerful individual who was well-loved by the people. Once their beloved mother had left the world to meet their father, Kurt spoke to Ren. 

"Brother, I hope to go and have my own adventures. I wish you would give me your blessing and allow me to leave the continent. I want to explore the great blue ocean and see what lies beyond the horizon." 

Ren was already expecting Kurt to say this since the only reason he stayed for so long was because of their mother now that she's gone there was nothing stopping him. 

"Alright, I'm going to agree, under one condition, and as my brother, you should already understand what it is."

Kurt took out his sword and so did Ren, the two smiled at each other and started attacking. 

...

A couple of years after Kurt left to have his own adventures, Ren was now the father of many children. He had married Tsukiko, Henrietta, Hilda, Lara, Silika, Tsukiyomi, Leila, and Ashley. All of them had given him children, who were close in age. 

All of his children were born into this world with large amounts of mana, and each one was a genius that went beyond common sense. Their skills and knowledge were different and though all of them were good at most things, they had their own specialized skills. 

They took different interests and pursued those interests to the extreme. Ren was delighted in seeing his children growing up as such. It was different from the time he was still Demon Lord Kretos, back then his purpose of having children was to challenge them once they grew stronger, but now he had them because of his love for his wives. 

...

It wasn't just Ren who was getting lucky with the ladies, but so was Valdel. While he was training to get stronger, by his side were Rachel and Natasha, and of course, his weapons spirits Nika and Zwei. After not responding to their feelings and ignoring them, the four women had made various plans to seduce Valdel. 

Being on the receiving end of their seductions day in and day out, even someone like Valdel had finally given in and laid a hand on all four ladies. After taking each of their first times, as a responsible person Valdel promised to marry them all, which he did. 

So like Ren, Valdel now had four wives and six children. 

...

Ren who watched his family and empire grow was once again getting a bit bored of waiting for Valdel. So he created a tournament for the young geniuses to showcase their talents. The winner of this tournament would then get the chance to fight the strongest warrior in the continent which was, of course, Ren. 

Over the years this tournament Ren created grew in popularity and become the most-watched event every year. The young geniuses within the entire empire would gather every year to show their peers that they were the strongest of their generation and in the end, all of them had the dream of one day defeating Ren to claim the number one spot as the strongest warrior. 

...

Fifteen years had passed since Ren had defeated the allied kingdoms and now it was the month of the yearly tournament to begin. The empire that started out from a little city called Grenton has now grown majestically. As for New Grenton, it has now turned into a metropolis that every single person dreams that they could live in it.