Demon Lord's Reincarnation

Erste did not hesitate as he killed the nearby New Gods. Unlike before it was no longer hard for him to kill New Gods as his mana and his control of it had increased dramatically, not to mention that he had gained many more authorities at this point in time. Even though he still could not harness the power from the void what he had was enough to win. 

After beheading the nearby New Gods, the remaining New Gods with some of the weaker Old Gods started to flee while the remaining Old Gods transformed into their true forms. Seeing them transform into their larger forms made Erste smile that vicious smile of his. 

"Heh, finally I can get to test out the martial art I created to deal with you guys." 

...

In the era Erste was in humanity did not even understand the bare minimum regarding mana and other things such as elemental changes. The only thing they used mana for was to strengthen their bodies and create runes. The only other form of attack they knew was to use the authority they stole from the Gods which they were unable to wield properly. 

There weren't any spells, there weren't any proper techniques to increase mana except for slaying Gods. Humanity at this stage was helplessly in the dark, they needed to create their own techniques, and they needed to explore how to manifest mana that would later on be called a spell. Still, the advantage of this era that no other era humanity had, was that Erste was in this era.

...

After a few trials and errors, Erste understood a little bit about mana. In the past, he was taught that atoms make up everything, and after fiddling with mana he knew that mana was something that could potentially create and destroy atoms. 

After learning more about mana the first technique or you can even call it a first-generation spell that Erste created was a cloning spell. After manifesting his mana outward Erste was able to control well enough to create several bodies similar to his own. Each body is easy enough to control the only problem is when they get too many, so Erste usually limits the cloning spell with twenty clones. 

Erste's clones had swords created by mana as well, the clones then started using one of Erste's original techniques. The clones lifted their swords up to the heavens and with the power of Erste's mana coupled with his current understanding of the sword. A beam of pure mana burst out of each of the clones' weapons. This was Erste's technique called God Slayer. 

The beams of light sliced through the surprised Old Gods, those that survived attacked Erste's clones while the original was controlling his clones from a distance. 

'This is harder than I thought... After using God Slayer with my clones, it drained a considerable amount of mana. Still, despite the huge amount of mana drained the combination of clones and God Slayer was quite a good idea in a situation facing multiple Old Gods.'

Erste knew that he could not use God Slayer multiple times due to his limited amount of mana, so he created other simpler techniques to deal with these gigantic Gods. Erste manipulated his clones to use the second technique he created to deal with the Gods, [Mana String.]

Multiple strings created from mana dancing through the fingertips, these strings were almost unbreakable as long as the caster has enough mana it will never break. The clones waved their hands as if they were conducting an orchestra. 

The Mana Strings wrapped around the Old Gods and they were squeezed to death. Unlike God Slayer which had a high mana cost but high attack power, Mana String had a lower mana cost but higher requirements for control. 

Despite killing so many Gods Erste's strength was no longer increasing much even after pillaging the abilities of these Gods. It was at this point Erste was slowly realizing that the old way of getting stronger was no longer needed and he was actually growing stronger even without doing anything.

It took Erste several hours to finally kill most of the Gods now only one remained. Unlike the other Gods who were panicking or were in a state of rage, this one did not have any kind of emotional fluctuation. The unknown God who did not show his true form slowly approached Erste. 

"That was an amazing sight, killing so many Gods like that. I really do envy you, humans. You were created by the father of all the ORIGIN the one who created my own creator, an Ancient God. Not only were you created by the ORIGIN but you also hold a majority of the ORIGIN's affection."

"I think I heard the other Gods say the same thing, and like I told them I don't even know who the f*ck the ORIGIN is. The only thing I understand is that you Gods are just whiny babies that just want to be noticed."

"As expected of a human, not even understanding how blessed he truly is." 

"Hmph, I might not understand what kind of blessing I have, but I do understand that you're not only a whiny baby but a jealous one as well." 

"Hehehe, you have become an arrogant little human. Is it because you think you're strong since you have defeated some weaklings?"

"Not really, I've always been like this, and I'm not arrogant, I'm only telling the truth." 

"You really are an interesting human. You are strong, brave, determined, and based on what I've heard extremely lucky. On top of all of that, you have the privilege to be loved by the ORIGIN. Among all the humans I have seen you are the most blessed. Which is why you are the one I hate the most." 

The last words the Old God spoke sounded different as if there was the sound of a cat scratching on the wall, the sound of something ringing, the sound of wailing, the sound of despair. It was an uncomfortable sound that could not just be heard but it even affected one's soul. 

"Now human I will show you true despair. I'm not the same as the Gods you have killed, I am one of the oldest among the Old Gods, created by the very first Ancient God. I represent his endless jealousy, his envy." 

The human-shaped Old God started to transform and unlike the other Old Gods what he was transforming into was not a gigantic humanoid version it was something else entirely. The Old God was turning into a mass of what seemed to be sawn flesh and scales, he had long bat-like wings and tentacles all over. The Old God didn't look like it had eyes instead it had thousands of mouths with different shaped teeth inside. The surrounding light seems to be eaten by the monstrosity standing before Erste. 

At this moment seeing the true form of the Old God, Erste actually felt his head aching, and his heart started beating faster, he was feeling cold and had difficulty breathing while sweating profusely with a slight tremble in his hand. 

"Oh, you're still able to keep your sanity despite witnessing my true form. You are even better than most Old Gods who would be driven mad merely being in my presence."

The voice of the Old God was a mixture of different disturbing things. It was an unpleasant voice if you could even call it that. In response to his current situation that would bring despair and madness to almost any other human, Erste continued to smile that vicious smile of his as he pointed his sword at the Old God.