I appeared beside him. Before he noticed me I took out the Sword of Kusanagi. I used wind chakra on it making it was sharp enough to even slice through a diamond.
I tried to cut off his head, but he just barely managed to dodge so I sliced off his arms instead.
"You bastard! How dare you pull a sneak attack on me!"
I appeared behind him and touched his back. He fell face-first into the ground. This jutsu rearranges the body's signals. That means that the brain can't communicate with the body so he won't be able to control his body now. To undo this jutsu you need a high amount of concentration and intelligence. Both things I doubt he will ever have.
"What did you do to me!?"
I aimed my hand at his head.
A stream of lightning went right through his head, killing him. Just to make sure I cut off his head and pierced his heart.
'I should have done this from the beginning.'
I stored away his body and head in a scroll.
I appeared in my lab. I stopped using the Ten-Tails Mode and stored away the sword. I put his body and head on the table.
'Let's see why this guy gave me so much trouble.'
[1 week later]
'Damn these cheating bastards."
The technique they are using is a self-sacrifice technique. They burn their life force to forcibly increase their power. The reason why they still use it is even though it has such a big drawback is because they can regain their life force.
For them, it's kinda like chakra. When it's drained it will refill by itself, but if it gets completely depleted they will still die. It's their bloodline that gives them that ability. Their life expectancy is around 15.000 years.
I held a blood-red vial in front of me.
'So this is what's going to give me the same powers.'
I summoned a clone and lied down on a bed. He put a seal on me that makes me feel nothing. Even if I get sliced up I won't feel it, but I will stay conscious.
My clone put the vial's content into a syringe. He then grabbed my arm and put it into my vein. I used my chakra to sense what is happening in my body and I couldn't see a difference, but when I looked closer I could see that my bones were getting more dense and durable.
'Aren't they supposed to get destroyed and then new ones magically grow in their place? I guess I've read to much Wuxia light novels when I was back on earth.'
Once my bones were done, my muscles started to get denser and they also started to turn pink.
[1 hour later]
Everything was finally done and the clone removed the seal. I tried to stand up but I fell face-first on the ground.
'My body feels completely different, but luckily it doesn't look different.'
I slowly stood back up by grabbing the bed. I sat on the bed and took out a kunai. I tried to stab my hand with the kunai, but it broke.
'Some people are just born as cheats.'
I tried to stand up again and this time I managed not to fall.
I appeared in the training place and fell down again. I stood up again and started to slowly walk around and then I started running till I was an hour further and I was able to completely control my body.
'Alright let's see how this technique works.'
A faint purple aura started surrounding me.
I got send flying into the wall. Most of my clothes were blown off.
'This thing is more difficult than I thought.'
It's kinda like the Eight Gates, but instead of their only being 8 gates in their body, they have 13. I now also have 13 gates and my Chakra points are also in completely different places. Also with the Eight Gates, you need to open one after another. But with this technique, you need to open them all at once. You can't have just 7 open or something like that.
'Now let's try that again.'
'You've got to be kidding me! Let's go again.'
I was now lying on the ground completely n.a.k.e.d.
'Is there something I'm missing? I have the same body and I know how to use the technique from his memories.'
I took out a new set of clothes and put them on.
'I need to blow off some steam.'
I summoned a thousand clones and started fighting them with only taijutsu. After I defeated half of them I almost got hit on the back of my head, but without me realizing what was happening I kicked the clone away.
All the clones stopped moving. I stopped using any chakra sense and stopped using my visual prowesses (the vision you get when you use the Sharingan or the Byakugan).
"Come at me again."
They started attacking me again. This time I was getting totally pummeled. They were still using chakra sense and their visual prowess.
'I need to completely empty my mind and let my reflexes do all the work.'
I started slowly blocking more hits and hitting back.
[1 day later]
There were still 100 left. A clone suddenly appeared above my head using the Flying Thunder God Jutsu. I grabbed his leg and slammed him to the ground. I allowed them to use that jutsu so that I push myself even further.
The Flying Thunder God Jutsu is instant teleportation so reflexes or predictions are the most useful on it. I'm able to block and counter-attack with only using my reflexes. I believe that this is the hidden power of the Bunjiro Clan. That is why that guy was able to dodge my sword when I appeared beside him.
(AN: No this isn't Ultra Instinct. His body will only move on its own when the brain can't handle it. So he won't stop thinking like Goku does which makes it weaker than Ultra Instinct.)
I closed my eyes.
I got enveloped in the purple aura, but this time it stayed and I didn't get damaged. I swung my fist at the clones in front of me. They were at least 10 meters away from me, but I hit all of them and they got dispelled.
'That was just a casual swing and it already did so much damage.'
The reason why I'm able to do it now is because my reflexes have hit a certain level. I think that not even the Bunjiro Clan knew that they needed to rely on their instincts to use that technique. To them, it just came naturally.
I teleported to an empty dimension that I found in Momo's memories. This dimension is completely made out of stone.
I punched the ground with all my strength.
A huge crater appeared under me. And when I mean huge I mean 2x the size of the crater pain made in Konoha.
'How could such a cheat race exist? Does that mean that he didn't use his full power on me?'