Many people wonder what is in the eye of Zoro. We saw so many theories like Zoro’s eye contains Asura…As it is shown many times whenever Zoro uses any powerful technique like ASURA (some kind of red light appears from his left eye ) so it is assumed that his left eye possesses some kind of demonic power.
There are theories according to which the scar on Zoro’s eye contains the demon which resides inside his cursed sword Sandai Kitetsu…
Other theories suggest that the scar is made intentionally to master the ability to sense the things/people around. It looks like Kenbunshoku Haki is an ultimate ability of swordsmen to fight without using sight sense. So losing sight might be A KEY to AWAKEN the ultimate Swordsmen powers. Zoro might have intentionally cut his left eye to awake this ability.
The most intuitive theory is that Zoro simply got hurt during his training with Mihawk somehow. This makes sense given what we know of Mihawk’s tough training style and his immense power and Zoro’s stubbornness.
Anyway I think I found something interesting. During the break of chapter 597 ( just before the time-skip ), Oda showed us the new chara-design of Zoro and his future wound.
We can see that Oda hesitated on many wound like the big on his forehead. Maybe Zoro’s scar is hiding a very good technique, or at the very least a thrilling story, but there’s the possibility that it’s just a normal wound and there’s nothing special behind it.
What strikes me as interesting is its baffling coincidence to the one Dark King Silvers Rayleigh has over his right eye.
Of course, it is a well accepted fact that Zoro is to Luffy what Rayleigh was to Roger. So maybe Oda has drawn similarities to both.