As Rentaro was casted into the genjutsu, he noticed that the trees which were all around Souji and himself faded away slowly, and a small lake started to become visible on his right side. Rentaro took two shuriken out from his ninja pouch and held one in each hand. He threw one of them using his right arm towards the area where the trees disappeared. The shuriken stopped right where the trees were previously, showing that the tree was in fact still there. Rentaro next switched the other shuriken to his right hand and threw it at the lake. Instead of the shuriken sinking into the water, it dug into the surface of it, like if it were in dirt.
Alright, this genjutsu changed what I see. Instead of the trees which are all around us in reality, they are now gone, and a lake is now over here, where it wasn't before. Rentato pointed to the area where he saw the lake, and where the shuriken were.
I tested to see if this was a genjutsu by throwing two shuriken at the places where things disappeared and appeared, and the shuriken didn't pass through the area where the trees disappeared, nor did the shuriken sink in the lake. I was confident that those were the things which were changed in the first place either way, as I watched the trees disappear and the lake appear.
Rentaro brought the thumb on his left hand to his mouth, and carefully bit it, breaking the skin ever so slightly. A small amount of blood dripped from Rentaro's thumb, but it stopped fairly-quickly.
In battle, I could use this if the opponent covered their entire body with a dome to protect themselves. I could yell the name of some jutsu which does major damage to the environment while jutsu using this genjutsu. It could cause them to maybe try and jump on things or use the environment in ways which wont work because there would be nothing there. I could also theoretically get the opponent to believe that I could use wood release by yelling out the name of a wood jutsu and then changing the environment accordingly. A combination with this is to maybe create some real rocks which they trip on or something so the opponent believes that they are now walking/running/are present in a rocky area while they could be walking towards a lake without channeling chakra in their feet, allowing them to fall in. Thats a very elaborate combination, but there.
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Question: have I already learned Kai from when you posted it earlier? I dont think I have right?
Also, about this technique. In the chakra cost it says
"Chakra Cost: 10 (+5 if you use it double)"
What does the "double" mean in this case?