A flashback from a couple of months ago...
On the roof of the Butterfly Mansion, Goku was sitting, sharpening his sword. This memory is from after the fight against Daki and Gyutaro. Goku had just lost his right eye. While he was sharpening his sword, he heard a voice from behind. "My, my, always pushing yourself to the limit, huh?" said the voice. Goku turned and saw Shinobu. "Oh, it's you, Shinobu-chan..." he muttered. "Yeah, it's me. Mind if I sit?" she asked. "No, feel free to sit..." said Goku. Shinobu sat down beside him. "Why did you hug me earlier, without saying nothing?" asked the boy. "Because I thought you needed a hug, after all that fight," replied Shinobu. "Well, you were right..." muttered Goku. "Do you think this eyepatch ruins my looks?" he then asked. "No. On the contrary, it makes you tougher. Like someone who won against death itself," replied Shinobu. "Oh, fine, I'm glad to hear it..." said Goku. "Anyway, there's a reason why I came talking to you," said Shinobu. "What is it?" asked Goku. "It's about your demon side. Have you ever wondered if you'll ever be able to switch sides?" asked Shinobu. "Huh? What do you mean? Explain yourself better," said Goku. "Surely, let me explain better," said Shinobu.
"As we all already know, you're a hybrid. Half human, half demon. You don't have any characteristic of a demon, aside from infinite lifespan. But maybe, that's because you're in your human form. Have you ever heard the word 'transformation'?" asked Shinobu. "Transformation? Yeah, of course I know what it means," said Goku. "Fine. In my opinion, if you improve your fighting skills and get stronger, one day, you'll be able to switch into your demon form," explained Shinobu. "Huh? But I wouldn't be able to turn back into a human, and I'll turn hostile!" said Goku. "And that's where you're wrong. As I said earlier, you're a hybrid. You already are a demon, simply with human features. But if you manage to transform into your demon form, even only for a while, you'll be able to turn back into a human on will. I mean, if you'll be able to transform into a demon, you'll also be able to turn back into a human." Hearing those words, Goku scratched his head. Was it really possible? Could his demonic cells really turn him into a demon? And if so, what would the conseguences be? "If what you said is really true... How could I be able to transform into a demon?" asked Goku. "And especially, how the hell do you know all of these things even I didn't know about?" he added.
"I'm a scientist, of course I know more things than you," said Shinobu. "Anyway, to unlock your demon form, I think you have to awaken your demonic cells," she explained. "Right now, your demonic cells are inside you, but they're sleeping. It's as if they aren't part of your body. To activate, you have to do the same thing you did when that strange mark manifested on your forehead five days ago, but even more difficult. To awake your demonic cells, you probably have to relax your body, then the options are two: The first one is to eat demon flesh, the second one is to drink your own blood. Personally, I suggest the second one, because it's easier. If you're in the middle of the battle, you're probably already bleeding, right? Just drink that blood."
When Shinobu finished to explained, Goku was confused. "So, if really the possibility to activate a demon form exists, I have to eat demon flesh or drink my own blood?" repeated Goku. "Exactly, but you have to relax," said Shinobu. "I admit it, I know this because I stole one of your hairs which contained demonic DNA, so I had enough time to analyze it and explain this to you," explained the Insect Hashira. "But why?" asked Goku. "Your goal is to defeat Muzan Kibutsuji, right? Well, so is ours. I knew you were special. You're probably the only hybrid on the face of the Earth, so I had to understand if you'd be able to possibly activate your sleeping demon. I don't think you'll turn evil after, so you don't have to worry," explained Shinobu. "Fine, I'll train to try to achieve this demonic form, if it really exists," said Goku. "Good choice, Goku-kun. Now I gotta leave," said Shinobu getting up. When Goku remained alone, he wondered: How could he possibly awaken his demon form? He knew he had demonic cells, of course, but a demon form? He never thought about it. But if this were true, if he really had a sleeping demon side, then he had to awaken it, for the sake of everyone.
Another flashback from a month ago...
Goku was in the forest, training. With his sword, he slightly cut his arm and drank his own blood. But still, nothing happened. "How...? Maybe this demonic form really does not exist!" he said, frustrated. "Shinobu-chan lied to me, I'm just a human with demonic cells, and this makes me a hybrid, I don't have any demon form!" he exclaimed. "Or maybe... I'm not doing this properly. She said 'you have to relax', and I'm not relaxing at all. Furthermore, I'm still not strong enough to tank the possible recoil of this demon form, especially if I use the mark at the same time..." he realized.
From that day on, Goku never tried to awaken his demon form. Maybe he had lost hopes. Maybe he had forgot about it. Or maybe, the time ran out, since he went to the Swordsmith Village a month after. Surely, this demon form does exist.
Back to the present...
Goku took a deep breathe and relaxed. Everything around him disappeared. Then, with his only eye closed, he placed his mouth on his hand and drank his own blood. Suddenly, Goku felt a strange sensation washing over him. His eye changed color. His sclera became red, and his pupil turned yellow. His teeth sharpened, and his body felt as if all the damages Akaza inflicted him disappeared. In that moment, Goku's demon form awakened.
Meanwhile, Giyuu was still attacking. While trying to landing a blow, Akaza broke the Water Hashira's sword. (Shit, I'm done for!) realized Giyuu. "Goodbye!" exclaimed Akaza. Then, the demon punched Giyuu in the gut, piercing his stomach. But actually, he didn't. Akaza's hand got cut by someone. But not Giyuu. (What... What was that? Why didn't I die?) wondered the Water Hashira. Then, Giyuu saw him: Goku, in his new form. And strangely, Akaza didn't even notice him. (Goku just saved my life...) realized Giyuu. (He's... different. Very, very different. Why did his eye turned red? And why does he smell like a demon?) wondered the Water Hashira. "Akaza!" shouted Goku, catching Akaza's attention. The demon turned. (Baka, don't call him!) thought Giyuu. "Now I'm gonna behead you!" exclaimed Goku. "Oh, your appearance has changed! I like it!" said Akaza, preparing to throw a punch to the Sun Hashira. "So you eventually accepted my proposal to become a demon!" he exclaimed, releasing the punch. But like it ain't nothing, Goku dodged, and before Akaza could react or even notice, Goku countered.
"Sun Breathing: Second Form -- Clear Blue Sky." His sword in his right hand moved faster than anything the Upper Rank had ever see. Then, a strange sensation. Akaza's head slowly started to disattach. (He... He beheaded me! But how? I didn't even saw it coming!) wondered Akaza. With both of his hands, Akaza blocked the fall of his head and slowly began to reattach it. "He's trying to reattach his head!" exclaimed Goku, with blood descending from his forehead. But then, as he tried to get up, his body stiffened. (Ugh! This is the recoil for using the demon form, even if I used it for a few seconds!) realized Goku. Fortunately, Giyuu threw his broken sword towards Akaza. It revealed to be a good decision, as Akaza's head fell on the ground. (No... It can't end like this... I have to become stronger!) thought Akaza. (His body didn't fall... What does it mean?) wondered Giyuu. Suddenly, without apparently reason, Akaza's body continued to move. It gained the demon's usual fighting stance, and without hesitating, it attacked Goku. Fortunately, the boy rolled aside and dodged. (The wound on the neck closed! How can this be possible?) wondered the Sun Hashira. Then, another kick arrived. This time, Goku couldn't manage to dodge, and he got smashed against a wall. (Even if I beheaded him, he's not dying! Is there a special condition like Daki and Gyutaro? No, this can't be, the head disappeared... Akaza is trying to transform into something else!) realized the one-eyed boy. (Just like Muzan, he's trying not to make his head his weak point! This is not over yet, I have to...) but before he could finish the phrase, Goku fainted from the recoil of his demon form and injures.
(He's surely fainted, he pushed himself too far...) thought Giyuu. Then, Akaza's body walked up to Goku. "Water Breathing: Fourth Form -- Striking Tide," said Giyuu, quickly cutting Akaza's chest. But the demon regenerated. (He's still regenerating, even in this conditions!) thought the Water Hashira. Then, Akaza punched him and Giyuu stepped back a few feet away. Then, the demon's body returned to aim Goku. "Wait, I'm still alive! If you want to kill Goku, you have to kill me first!" said Giyuu. (The situation is getting worse... I'm about to faint, and I can't hear nothing from my left ear... I still have strength, but for how long?) he wondered. (I won't allow nobody to die right before my eyes anymore. I'll save him. I'll save Goku, no matter what...) he thought. Suddenly, when Akaza was about to strike again, someone grabbed his hand. But he didn't recognize who they were. "Let go of my hand," he said. The one who was holding his hand was a girl. "Let's go away," she said. "No. I have to kill them," said Akaza. "Why?" asked the girl. "Because I have to become stronger... To give my father that medicine."
Chapter 56 - End.