Ren Tian You sat cross-legged inside the hole, and recovered his condition to optimum state, and he was immersed with the system's interface which was inside his mind and prepared to awake Mangekyo Sharingan.
Mangekyo Sharingan is the powerful mode of Sharingan, awakening this eyes not only increase the power of eyes, but also, each eyes contains formidable eye techniques.
However awakening Mangekyo Sharingan not only gave powerful eyes techniques, but the eyes will also receive a kind of curse. Mangekyo Sharingan varies with the user, and its excessive use can cause decline in vision and step by step lead towards darkness, and after that blindness.
Only those peoples who had awakened Eternal Mangekyo Sharingan, can break this curse of Mangekyo Sharingan, as well as using those eyes techniques won't lead then towards blindness, and their eyes would have eternal light. Uchiha Madara had used the ability of this Eternal Mangekyo Sharingan to conquer various tribes, leading Uchiha clan to achieve most flourishing time. And became one of the two most powerful clan along with Senju clan.
In Naruto world, there was only 6 people from Uchiha clan who had awakened Mangekyo Sharingan. They were Uchiha Madara, Uchiha Izuna, Uchiha Shisui. Uchiha Obito, Uchiha Itachi and Uchiha Sasuke. [Note: Don't talk about anime fillers ok, in manga only these people got Mangekyo Sharingan. (Opinion differs, fell free to comment your opinion.)]
And in this Naruto system, Mangekyo Sharingan is based on these 6 peoples to select. And according to the system, he could select to awaken all 6 of them as long as he got enough system points.
He didn't have enough system points to open all 6 of them, and he also didn't know whether he would have the curse of blindness after awakening Mangekyo Sharingan. He had already made his decision for whose Mangekyo Sharingan to awaken first.
The first Mangekyo Sharingan he would awaken was of Uchiha Itachi.
Because doujutsu (eye technique) of Mangekyo Sharingan of Itachi was more comprehensive in comparison. With his left eye, he could use Tsukuyomi, a powerful genjutsu (illusion technique) that could trap opponents' minds in an illusory world, could alter their perception of time to make seconds of torture seem like days and he could even magnify the pain sensation. And even more frightening was, the opponent couldn't tell when they fell in the illusion technique and after falling in the illusion technique, they couldn't tell the difference between reality and illusion. And he could use this technique to kill the opponent by having them experience decade's worth of time in a fraction of time too. This is very dangerous and powerful doujutsu. This doujutsu could be avoided, only when the opponent didn't make eye contact with the user of this skill and Tsukuyomi could only be broken by a Sharingan user that shares the same blood as user (this was stated by Itachi). (In original version, although Kakashi had Sharingan, but he was not the member of Uchiha clan so he was unable to resist Itachi's Tsukuyomi.)
With his right eye he could use Amaterasu, It is the most powerful firejutsu which could igniting whatever he looked at with inextinguishable black flames that would burn anything, including fire itself. The range of this technique was large and average ninja couldn't avoid it. (Of course, raikage with abnormal speed and Gaara with abnormal defense are not counted among average ninja, and also Pain who could easily flick anything with Shinra Tensei.)
And for other person's Mangekyo doujutsu, Sasuke's left Mangekyo also possessed Amaterasu, and with his right Mangekyo he can shape the flames or extinguish them. His Mangekyo lacked godly attack like Tsukuyomi. As for Uchiha Shisui's Mangekyo, although it had extremely powerful genjutsu which was capable to the extent of creating miracle, but it had extremely big weakness, which was, the time interval to use it again was too large. If he didn't possess the first hokage's cell, it would take 10 years of time to use that ability again. Although Uchiha Obito's Mangekyo possessed Kamui ability, but it inclined towards space. Grandfather Madara's Mangekyo possessed ability inclined towards time. And Uchiha Izuna's Mangekyo's ability was unknown. So he had decided to awaken Mangekyo Sharingan of Uchiha Itachi first. [Note: We don't know the ability of Madara's Mangekyo, I had just translated what author had written.]
The person who had awakened Mangekyo Sharingan in both eyes can use ultimate doujutsu— Susanoo. He can summon a gigantic, humanoid being made of the user's chakra that surrounds them and fights on their behalf. And it can also be equipped with weapons by the users. Itachi had equipped 'Sword of Totsuka', 'Yata Mirror' and curved blade. Sasuke had equipped with bow in left hand and arrow in right.
"System, I want to exchange Mangekyo Sharingan of Uchiha Iachi." Ren Tian You sunk his consciousness to his mind, and there towards the interface of system, he spoke.
"Yes host. Awakening Mangekyo Sharingan of Uchiha Itachi consumes 10,000 system points. Now only 673 system points remains." He heard the system voice in his mind, followed by this he suddenly felt a wave of terrible pain between his eyes.
"Ah!" He roared crazily, he felt severe pain like thousands of ants was nibbling him, and he couldn't even resist or endure this pain.
Inside the hole of the tree, he released heart-breaking roar, even though he had gone through more than 2 years of training and become powerful but he wasn't able to endure a bit of this pain. After some time, he clearly felt that a clear stream was flowing towards his 2 eyes, and he started to feel crisp pain in his 2 eyes. He felt so much pain that he covered his eyes with his hands.
Still he was exerting himself to bear this pain, he knew that this was a big test, as long as he bypass this pain, then his strength will rapidly increase and that was a step closer to find a way back home.
After about 10 minutes, slowly the pain in his eyes started to subsided, and instead came the surge of comfortable feeling, which couldn't be described in any words. In this 10 minutes, his was almost hoarse because of screaming in pain. At this time, system notification sound rang.
Ding: Congratulation to host for awakening Mangekyo Sharingan with the side effect of making user blind. Now you can awaken Eternal Mangekyo Sharingan with enough system points.
Along with the sound of system, he had succeed in awakening Mangekyo Sharingan. He slowly loosened the hands that was covering his eyes, raised his head and looked all around him. He clearly felt the changes Mangekyo Sharingan had bought in his vision, he could see all the things even more clearly and all the movement and movement paths also became incomparable clear.
"So this is the power of Mangekyo Sharingan, normal Sharingan could never compare with it." As the world he looked became different, he told himself.
Then he came out from the hole and head towards small stream and looked at the changes in his eyes. For sure, his eyes had design of Itachi's i.e. three spiraling curves around the pupil.
After observing for a while, he opened system interference, and started to look for changes in the system. At once he saw the doujutsu system record. Awakened Mangekyo Sharingan, using rises blindness value, blindness value is 0% now. If you open Mangekyo Sharingan and not use its doujutsu, then that will not rise blindness value. If you use either Tsukuyomi or Amaterasu, this will rise blindness value to 1%. And use of Susanoo also rises blindness value by 1% per minute it is sustained, and less than 1 minute also rise blindness value by 1%. When your blindness value reach 30%, your vision starts to blurred, at 60% your vision becomes like the vision of people on the Earth with 800 degree of myopia and 100% is permanent blindness. Blindness value could be offset by using system points, 50 system points per 1% blindness value. Unless you awakened Eternal mangekyo Sharingan, this blindness side effect couldn't be removed. And the good news was the simple side effect of Mangekyo Sharingan such as, eyes bleeding, pains and aches and so on were all automatically removed by system. As a result, unlike in original world, there will not be any uncomfortable after using Mangekyo Sharingan.
And he had to awaken Eternal Mangekyo Sharingan for all Mangekyo Sharingan. For example, he had Itachi's Mangekyo Sharingan, now he could awaken Eternal mangekyo Sharingan for current Mangekyo if he had enough system points. And again if he awakened Sasuke's Mangekyo, then for this too he had to awaken Eternal Mangekyo and so on. And it required 100,000 system points to awaken Eternal Mangekyo Sharingan. An astronomical figure.
By looking at these information, the happiness he got by awakening Mangekyo of Itachi reduced to half. He showed a wry smile.
"Ai, it seems for the sake of awakening Eternal Mangekyo, I would be very busy. I also want to awaken Rinnegan, oh, how I feel this system is simply a bottomless hole." He gloomily cursed loudly.
He again sat cross-legged inside the hole of huge tree, and slowly adapt to the new power of his Mangekyo Sharingan. After nearly half a day, he had adapted with his new powerful strength. It was already late at night and there were howls and roars of magical beasts coming from inside the magical Beasts forest. He looked towards the 2 bright moons in the sky, again heard roars of magical beasts. An easy to read brilliant smile appeared in his lips.
"Beasts bastards, uncle I had accompanied you all for more than 2 years of savage life and tomorrow I'm going to leave this forest. Ai, how come I became a little reluctant and depress to leave here and when did I get accustomed to this savage life? Don't think, sleep and rest, Tomorrow I have to leave this forest."
He was surprised with his own sudden reluctant to leave heart, he shook his head, stopped thinking about it and lay down in the hole and fell asleep.