Demon Lord's Reincarnation

Ren who was standing on top of the walls of Grenton looked at the undead army and smiled that vicious smile of his. He summoned his death scythe and placed it on his shoulders. Silika was unable to talk at the moment since she was still trying to recover her strength. So for the meantime, Ren wasn't able to use some of the death scythe's abilities like turning kill count into mana. Even so as a weapon the death scythe was still one of the strongest out there.

"Everyone who doesn't want to die retreat back to the walls!" Hearing Ren's command all the human combatants except for Valdel and Lara retreated into the walls. Even the Regalcrags who would gladly die in the battlefield retreated since this was an order from their new liege. 

While everyone was trying their best to retreat, Ren jumped up and slashed downward at the nearby fortified scaling tower. The tower that was able to withstand a barrage of spells was easily sliced in half by Ren who then proceeded to kill all the ogre kings that were protecting the scaling tower. 

It took Ren nothing but one slash to slice the ogre kings in half. He then started to move around with his scythe, the spinning of the scythe coupled with his acrobatic moves made Ren look like he was dancing around the battlefield. 

The people who were watching on the walls were dumbfounded by what they saw. They almost believe that they were dreaming, some actually think that they're already dead and what they were seeing was simply a last-second vision before death. 

...

Ren was actually casually killing the waves of undead like he was simply harvesting crops on the field. The speed in which he killed the undead was so much faster than all of their combined efforts. 

The two holy maidens couldn't believe what they were seeing either, Ren wasn't using even a tiny bit of mana as he was moving around like that. Each move was so fast and powerful, that they couldn't understand how Ren was doing such a feat without the use of any mana. It would've been possible with divine blessing, but they could see that Ren wasn't blessed by any God they knew.

Galius, on the other hand, was more surprised by Ren's skills. The precision of his attacks and the efficiency in which he initiates them was amazing. Ren's fighting style was simple and direct, it had none of the usual complications of other styles, but it was as effective, nay in Galius eyes, it was near perfection. Galius wasn't sure as Ren wasn't using a sword, but he believed that Ren's skill should be higher with a sword, so it was entirely possible that Ren was already at the Divine level of swordsmanship. 

A fifteen-year-old divine swordsman was unheard of, this made Galius really excited. Even though he was wounded all over, he wasn't bothered as he wanted to watch every single move Ren did. 

...

The people who were confused as to how Ren was doing these kinds of movement without using mana, were mostly the mages since they knew how impossible it was for the human body to have such strength without the use of mana. The only people they knew that could do such a feat were people that had bloodline abilities, like the royal family of Reschbeauch, who gain inhuman strength in exchange for their inability to accumulate mana. 

So some of them were wondering what kind of bloodline ability Ren was using. But the truth was they were wrong to assume Ren was using a bloodline ability, when in fact he was simply using Spirit Aura. Unlike mana, Spirit Aura was much more effective in enhancing one's physical abilities. 

...

The Elder Lich was amazed by how Ren was able to easily sweep the battlefield. Wanting to see more of his abilities, the Elder Lich ordered all of his subordinates except for the dragons to attack Ren. 

All the undead that were attacking the walls of Grenton stopped and started to concentrate all their attacks on Ren.  Seeing that all the undead were surrounding him made Ren smile that vicious smile of his even wider. His eyes were practically gleaming in excitement. 

"Finally deciding to take me seriously! Hahaha! Come on then, satisfy my hunger for battle!" Ren at that moment finally used mana. He started to chant a very long chant as he continuously spun his death scythe attacking and defending at the same time. 

After he was done chanting, Ren using his death scythe started swinging at the ground drawing a magic circle. Once he was done drawing a magic circle, Ren stomped on the ground activating his spell. Spikes made from earth pierced through the undead that surrounded Ren. The spikes from a distance looked like a flower, and Ren was in the middle. With that one move alone, Ren was able to destroy half of the undead forces. Just one move and he was able to kill more undead than all the people present. 

It was as if he was mocking them for their wasted efforts. The holy maidens gulped as they witness the scene of Ren easily destroying the undead. That spell he used was a high-level spell, a spell that needed someone to be at least a court magician level mage to use. Even with a massive amount of mana, a person won't be able to use a high-level spell without understanding the formula behind it. It takes years of study to be able to use such a spell, yet here was a person under twenty who was able to use it. 

Not only was Ren a powerful close-quarters combatant, with skills that could put a Saint-level swordsman to shame. His control of mana and his ability to use and understand complex spell computations made him a powerful mage as well. 

...

Ren who was in the middle of the pierced undead continued to smile viciously as he pointed at Alfred who was still standing at the back of his army. 

"Is this it?! Is this all you have to offer?! No, it isn't right! I've finished warming up, so how about you come and fight me yourself, Alfred!" 

Hearing Ren's confident voice made Alfred cackle. He then snapped his fingers and the remaining undead except for the two undead dragons, turned into dust. Alfred then began to absorb the mana that he used to create this undead. Once he was done absorbing the mana, he teleported in front of Ren. 

"Very well, Ren, allow me to entertain you. This will be my thank you gift to you, for offering me your body for the sake of my research on immortality. After I'm done experimenting on your body, I think I'll turn you into an undead warrior. Rejoice you'll be my right-hand man."