Shinji had actually considered persuading Ōtsutsuki Isshiki to join the fight against the Overlord, but he quickly dismissed the idea.
He knew that any member of the Otsutsuki Clan who had taken part in plundering planets would never feel compelled to oppose the Overlord. They were all "plunderers," and understood better than anyone how powerful the Overlord was.
And Ōtsutsuki Isshiki was exactly that kind of Otsutsuki!
To him, the Shinobi World was nothing more than an ordinary hunting ground. He had no feelings for it and no ties to it. So Shinji figured that if he told Isshiki about the Overlord's invasion of the Shinobi World, Isshiki wouldn't help him fight the Overlord. He would use the information against Shinji instead.
Shinji couldn't take that risk.
Beyond that concern, Shinji made no secret of his desire for Ōtsutsuki Isshiki's Black Eye.
Isshiki's Black Eye was a Kekkei Genkai of the eyes, entirely independent of Kaguya's lineage and unlike any Kekkei Genkai found in the Shinobi World.
That meant that if Shinji obtained the Black Eye, his Susanoo Lamp could use it to greatly supplement its Dojutsu properties. Then, wielding the Black Eye would give him an unprecedented boost in Dojutsu power.
There was another very important reason why Shinji had to deal with Ōtsutsuki Isshiki.
It was also why Kaguya Otsutsuki had ambushed Isshiki all those years ago: if someone wanted the Divine Tree to produce a Chakra Fruit, they had to sacrifice a true Otsutsuki to it!
Back then, Kaguya Otsutsuki had sacrificed Isshiki and obtained a Chakra Fruit. But she had underestimated his ability to cling to life. She hadn't expected him to survive by implanting himself in the form of a Karma onto Jigen, a monk who happened to pass by.
"Well, I already helped him