Control or fusion?
Shinji was startled at first, then quickly realized that his Susanoo Tomoshibi might allow him to bypass the surgical process and use dojutsu to fuse the powers of both pairs of eyes.
At the same time, this gave him a clear understanding of the distinction between "control" and "fusion" in the context of Susanoo Tomoshibi.
These were two entirely different concepts!
"Control" referred to using the second pair of empty eye sockets to accommodate a second pair of eyes, thereby gaining part of the eye's dojutsu or some of its powers.
This was Shinji's current state.
Usually, in the control state, one cannot access the full power of the second pair of eyes.
This made sense—after all, the second pair of eyes belonged to someone else, and a certain amount of rejection or incompatibility was inevitable.
How much power could be drawn from the second pair of eyes depended on the strength of one's own eyes.
For example, since both were Mangekyō Sharingan, Shinji could only manage to draw about fifty percent of the power from Fugaku's Mangekyō Sharingan.
If Shinji possessed an Eternal Mangekyō Sharingan, he could harness about seventy percent of the power of a regular Mangekyō.
Even further, if one controlled a Mangekyō Sharingan using a Rinnegan, one could potentially extract eighty to ninety percent of the Mangekyō's power, approaching full control.
"Fusion," on the other hand, as the name suggested, meant merging the two pairs of eyes into one, completing the fusion of their powers. At that point, only one pair of eyes would remain.
Every fusion resulted in some loss—this was a natural law.
Just like in the original timeline, when Sasuke fused Itachi's Mangekyō Sharingan with his own to gain the Eternal Mangekyō, he couldn't fully inherit all of Itachi's dojutsu.
Tsukuyomi was lost in the process.
Similarly, Susanoo Tomoshibi's fusion had limitations.
However, Susanoo Tomoshibi had a unique feature: the degree of loss during fusion was determined by the strength of one's own eyes.
Using a Mangekyō to fuse with another Mangekyō would result in a similar loss as surgical fusion, with a high chance of losing the dojutsu of the fused Mangekyō.
If one's own eye power was weaker than the other, there could even be a risk of fusion failure.
On the other hand, if one used an Eternal Mangekyō to fuse with a regular Mangekyō, the loss would be smaller, reaching an acceptable level.
The stronger the eyes, the smaller the loss during fusion.
If one's own eyes reached a certain level of strength, they could even attempt to fuse with other bloodline dojutsu, such as the Byakugan!
After understanding these aspects of Susanoo Tomoshibi, Shinji didn't rush to make a decision. Instead, he deactivated Susanoo to avoid unnecessary depletion of eye power, then sat quietly in the cave to contemplate.
From the nature of Susanoo Tomoshibi, it was clear.
The stronger one's own eyes, the greater the options, whether for control or fusion—this held true in both cases.
Therefore, the best course of action was to enhance his own eye power.
Moreover, Shinji had already noticed that after controlling Fugaku's Mangekyō, his own eye power had increased significantly, surpassing that of a normal Mangekyō Sharingan.
However, he still couldn't replenish his eye power by refining Yin Release chakra on his own.
This meant his current eye power level was somewhere between an Eternal Mangekyō Sharingan and a regular Mangekyō Sharingan.
"It seems the right path is to first fuse into an Eternal Mangekyō Sharingan, so I can autonomously restore my eye power!"
Shinji formed a plan in his mind, though his expression grew serious.
After using Susanoo Tomoshibi to control Fugaku's Mangekyō Sharingan, he not only gained more insight into the technique, but also learned important details about Fugaku's eyes.
One of these details troubled him greatly.
Fugaku's Mangekyō Sharingan had more eye power than his own!
This meant Shinji was actually controlling something stronger than himself.
"His Mangekyō, awakened for seven or eight years, still holds more eye power than mine, which is only a few weeks old... How did he manage that?"
Shinji was speechless.
He knew he had been using his Mangekyō Sharingan a bit too recklessly lately.
But Fugaku had awakened his Mangekyō during the Third Great Ninja War, which was before Sasuke was even born.
Yet, after all these years, Fugaku's eyes still had significantly more stored power than his.
"This guy really knew how to conserve his energy!"
The judgment of Susanoo Tomoshibi was not wrong, so it was clear that Fugaku had barely used his Mangekyō Sharingan since its awakening.
It was highly likely that throughout his entire life, Fugaku had never even once used Susanoo.
Shinji furrowed his brows slightly.
When he first considered fusion, Susanoo Tomoshibi had immediately given him a warning.
The chances of fusion failure were high due to the imbalance of power!
This helped him understand why, throughout history, only blood relatives had successfully fused their Mangekyō Sharingan into an Eternal Mangekyō Sharingan.
Uchiha Madara and Uchiha Izuna, as well as Uchiha Sasuke and Uchiha Itachi, were prime examples.
Because only blood relatives had matching chakra properties, their eyes wouldn't reject one another during fusion.
Although Shinji and Fugaku belonged to the same clan, they were not direct blood relatives.
The chakra properties embedded in their Bloodline Limit were clearly different, meaning Fugaku's eyes would instinctively resist fusion.
If their roles were reversed and Shinji's eyes were stronger, Susanoo Tomoshibi could force the fusion of Fugaku's eyes.
But in this situation, with Fugaku's eyes being stronger, Susanoo Tomoshibi could only help him control them.
Shinji closed his eyes and quietly focused on his own Mangekyō.
In fact, Susanoo Tomoshibi's control was a mild form of fusion.
For instance, when Shinji began controlling Fugaku's eyes, the loss of eye power in his own Mangekyō had partially recovered due to external stimulation.
He estimated that while controlling both pairs of Mangekyō, his eye power had increased to about 1.5 times its original state.
However, since he didn't know if this recovery effect could be triggered again, it was unclear whether he had gained any lasting ability to restore his eye power.
Additionally, perhaps due to his own limited use of dojutsu, Shinji had unexpectedly gained access to both of the dojutsu stored in Fugaku's eyes.
One of them, in Fugaku's left eye, was Amaterasu—just as he expected.
This suggested that both Itachi and Sasuke having Amaterasu was not only a reflection of their own hearts, but also an aspect of their shared bloodline.
But when Shinji discovered the Mangekyō Sharingan technique stored in Fugaku's right eye, his expression became rather strange.