Five-hundred years ago, during the Sengoku period, two little kids born. A woman, Akeno Tsugikuni, held two kids in her arms. Near her, there was the patriarch of the Tsugikuni family. "How should we call them?" asked the patriarch. "I think we should name them Yoriichi and Michikatsu. These names fits them, right?" answered Akeno. "They're twins. They're seen as bad omens, you should know" said the patriarch. "I know. But I do not care. They are my children, and I will protect them" said Akeno. "One of them has a strange mark on the forehead" said the patriarch, pointing his finger on Yoriichi's forehead. "Maybe it is a good sign. Don't judge him too quickly" replied Akeno.
Years passed, and Yoriichi and Michikatsu grew up. However, Yoriichi's Mark made him incredibly powerful. One day, Yoriichi and Michikatsu were sitting on a rock, as Yoriichi spoke up. "Hey, brother... Have you ever think about how would it be to fight demons?" he said. "What do you mean, Yoriichi?" asked Michikatsu. "I mean... I don't know what you think, but I do know I'm tired of hiding from demons. I want to fight them" replied Yoriichi. "Are you sure about that? I mean, I'm in, but what if we die in the process?" said Michikatsu. "You're forgetting I have the mark. This means I can protect you if you're on the verge of the death, whether you want it or not" said Yoriichi. "...Okay. I'm in. Let's train to fight demons" ultimately said Michikatsu. And so, Yoriichi and Michikatsu started training, and they became the first two demon slayers in history. "How should we call the ones who fight demons?" asked Michikatsu. "Demon Slayer Corps. Does it sound good?" proposed Yoriichi. "It does" replied Michikatsu. "But swords alone won't be enough to fight more powerful demons. We should create something to fight them better" said Michikatsu. "What if we create Breathing Techniques? A technique that you can achieve by simply breathing. So it'll be easy for the slayers that'll come after us" said Yoriichi. "Breathing Techniques... I like it" said Michikatsu.
Other years passed. Yoriichi grew up. Due to his superiority in strength, Michikatsu became envious of him and became more distant. One day, Yoriichi met Uta, a young woman with beautiful white hair. For the first time in his life, Yoriichi fell in love with a woman. Eventually, they married and had a family. They even had a kid. "Yoriichi... How should we call the baby? It's a boy" asked Uta. "Let's call him Goku. This name means 'The one who enlights the void'. I'm sure that one day, he will do what his name means" said Yoriichi. However, not even a month after Goku's birth, Uta was killed by a demon. "Muzan went too far..." said Yoriichi to himself. "This time, he's gonna die by my hands". And so, enraged, Yoriichi reached Muzan inside the Infinity Castle, Muzan's residence. As Yoriichi walked inside the Infinity Castle, he killed the demons that got in his way and eventually walked in front of Muzan Kibutsuji, the progenitor of all the demons. "And so, you eventually came, Yoriichi..." said Muzan. "I did. Killing my wife was the last straw" said Yoriichi, with an unusual rage in his eyes. "Do you plan to kill me?" asked Muzan. "Yes I do" answered Yoriichi. "Fine... Let us see how long you last..." said Muzan. Despite his boldness, Muzan got defeated by Yoriichi, who needed thirty seconds to humble the Demon King. To avoid death, Muzan splitted his body into pieces. "I didn't kill him... But at least, I made him realize he's not immortal..." whispered Yoriichi. As he slowly sheathed his red katana in his pocket, Yoriichi reached a female demon, named Tamayo, and freed her from Muzan's control. Thus, Tamayo became the first good demon. One month passed, and Yoriichi took care of Goku alone, after Uta's passing. But he quickly realized that, after his fight with Muzan, he was the main threat to demons, and decided to protect at least his son. "It's a hard decision... But I will give my son to Tamayo..." said Yoriichi. And so, with his son in his arms, Yoriichi made his way to Tamayo's place.
In the night, Yoriichi reached Tamayo's place, and knocked twice at the door. After a while, Tamayo opened the door. "Hello, Yoriichi. What brings you here?" she asked. "Greetings, Tamayo. I'm here because I made a hard decision" replied Yoriichi. "This is Goku, my one-month-old son. He's the most precious thing I have in this world. But this is the reason I have to give him to you. If I keep walking around with him, he'll surely be killed by Muzan or any other demon. I can't allow it. So please, take my son with you and protect him" he explained. "Why are you giving him to me?" asked Tamayo. "Because you're the only one I trust" answered Yoriichi. "Is there anything more I have to do?" Tamayo asked. "Yes. Once I leave, make sure he remains a baby until enough time has passed" he explained. Then, Yoriichi extracted a strange serum. "This serum will turn Goku into a hybrid. Half human, half demon. He won't be able to regenerate or have unlimited energy, but he will have immortality and be immune to sunlight. This serum contains cells from a demon I purified. I want to turn my son into a hybrid because I'm sure that one day, he will defeat Muzan, so making him live forever is the best choice. He is also born with the Mark, just like me". As Yoriichi injected the serum on the sleeping infant, he continued to explain. "He doesn't have to know anything. He won't know he's my son, or that he's half demon, or that he has the mark. Once I die, wait five-hundred years. Then, abandon him, and he will live his life properly, as a ordinary human. But for this time, he has to remain a baby. Can you do that?". As Yoriichi finished the explanation, Tamayo narrowed her eyes, but ultimately nodded. "I understand, Yoriichi, I will do so" she said. Then, Yoriichi left. Five-hundred years later, Tamayo was walking inside a forest, holding the still infant Goku in her arms. "I managed to keep him a baby thanks to my potions, but now... The time has come" said Tamayo, placing Goku in the middle of the forest. "May your life be great, and I hope one day, you'll discover everything and come back to me". And so, Tamayo left Goku in the forest. He was later found by a couple of merchants, who adopted him. And so, the story of Goku Tsugikuni began.
Prologue - End.