Alas you have suffered, in all your life the plan you have painstakingly worked upon ended in failure, there was no plot twist, just mindless training and fighting, there were no unexpected events, no surprises.
You have overlooked one thing, is that the villain was too obsessed with the hero that killing him was not enough, to vent the villain's frustration he even hung the head up the wall and covering it with a viel.
When the hero was crowned as the hero of the kingdom by the king for ending the war alone in the battlefield with twenty men, the hero headed to slay demon king but the hero fails but was let go by the demon king for deeming him weak and were thrown out, alone, his comrades all dead, even with him and his men alone wiped out all the demons, infront of the demon king, it was instant defeat.
Next when the hero was heading to a place to recover, he sensed a persons life signature nearby and made a stance, when the figure walk up close he was shocked and before he can spout a word, the figure with a face mixture of displeasure and disappointment was shown before the figure blurs and delivered a blow.
Only if it wasn't for the hero's injuries from the demon king, the hero could defeat his opponent easily, but now there was no time to think, right now there was only a fight to death.
In the end the hero still lost, he was too weak at his current state to even unleash any of his strongest attacks, before he closed his eyes a flash of thought and he remembered a distant memory, a promise between two childhood friends.
Because of one mistake, the other party fought him and tried to kill him on the spot, that's right, the promise of his childhood friend of meeting each other in the future and marrying each other, because of this one mistake, the direction of his life changed drastically.
For the other party mistaken him as a girl when they were both children in the past.
Before contemplating, you watched in disappointment at the show before you, the demon king is just too strong, impossiblento beat, he is unbeatable, the only invincible existence in this world that shake the very heavens to the point of the gods cutting their connection to this world, that alone points out how terrifying the demon king's power is, but being that strong the demon king stayed up in his castle for all his life and never left.
For that reason, the humans always pointed out all their problems to demons, especially the demon king, never fearing the demon king for he never left his castle.
Now, to change the fate of the hero and the antagonistic villain who showed up late with depression, you started performing the greatest magic you invented.
A magic that rewinds time, a fate changing magic that even the demon king cannot even grasp.
So time turns back to the past, at this time you just reincarnated to this novel, the one novel you read before accidentaly falling into a manhole and died.
But before you could implement a new plan, you were suddenly inside a castle, the demon king sat on the throne and look at you smiling,
you were scared as to why this happen, how could the demon king know you at this time, it doesn't make any sense.
You were questioning yourself, prostrating to the ground at the happy demon king, but you cannot stop now, so you tried going for a punch to the demon kings chest to which does nothing like you expected.
Doing this the demon king paused and his smile growing more bigger, you are afraid but to witness the drama between your favorite novel characters you have to survive from the demon king.
And it seemed to be working as you heard the demon lord left a sentence to the air for you to hear before you walk outside gasping for air.
It seemed to be something like grow stronger and kill the demon king.
So you go back to your plans and after trying all sorts of plans you had in mind it all ended up in failure, again.
Rewinding time and time again and again, it seemed that after the first encounter of the demon king, you weren't directly teleported inside his castle anymore.
Realizing all you have done ended up in failure, so you planned the only way for the hero to survive after failing to aubjugate the demon king from the villains hands.
And that is, to change the body of the hero, his traits of having the face, voice and body of a cute girl when he was a child, it all came to you now.
To change the direction of the story, to open the new path to another ending you have failed to envisioned.
This time, after the rewinding of time, you concocted a potion, a potion that change the body to the opposite sex, ita effects have no backlash apart from enheriting the way female bodies operate and how females react to physical contact.
With this it changed, a new plot unfolded in your eye as the hero and the villain falls inlove after a reunion, little did you know, your happy ending does not go by your plans.
The hero and the villain remembered all their past lives, every timeline you rewinds to the past, they both remembered and the hero has become a yandere, the villain becomes and obsessive psychopath, hiding a knife ready to strike against each other.
The hero then on abandoned his mission to subjugate the demon king as a monument of the previous civilization were uncovered and a tablet recording the demon king, the last creation of the god of this world made to become the last boss and justified power with almost impossible to beat.
Killed the god of this world, its creation after having fulfilled its role after creating the world.
The final trial of this world, the last boss.
But a last boss is a last boss and it will never interfere with the world, without the intention of doing anything besides waiting for an opponent that can end its role and die.
.....
Inspiration—webtoon; akari the final boss girl, character: Akari/Avalon.