When the Ketsuryūgan was activated, it looked very much like the Sage Mode of Mount Myōboku. Its pupils took the shape of a black horizontal line, while the whites of the eyes turned blood-red.
If the Red Eye was the natural enemy of the Byakugan, then the Ketsuryūgan was the mortal enemy of the Sharingan!
That was not quite right. More accurately, the Chinoike Clan, wielders of the Ketsuryūgan, and the Uchiha Clan, wielders of the Sharingan, were mortal enemies!
Moreover, many of the Ketsuryūgan's abilities were actually similar to those of the Sharingan.
Both dojutsu were renowned as "hypnotic eyes" and were exceptionally powerful in the field of genjutsu.
Beyond that, the Ketsuryūgan could turn humans into "Human Bombs" and could even use blood as a medium to cast all kinds of ninjutsu.
What was even more remarkable was that the Ketsuryūgan was awakened in the same way as the Sharingan—through intense emotional stimulation.
There was a legend that the Ketsuryūgan might have originated from Indra as well. Even their awakening methods were so alike that it was hard to believe the two had no connection.
Of course, all of that was merely legend. Whether it was true or false could no longer be verified.
But why were the Uchiha Clan and the Chinoike Clan said to be mortal enemies?
That went back a long time. The Chinoike Clan had originally been a shinobi clan within the Land of Fire, but they were defeated by the Uchiha Clan and ultimately exiled to a place called Hell Valley.
It was said that Hell Valley's environment was too harsh, and the Chinoike Clan could no longer produce descendants.
For all intents and purposes, they had been wiped out!
As for whether the Chinoike Clan had truly perished for that reason, and