After the time skip, we saw Zoro coming back with a scar on his left eye. At the beginning everyone thought he had lost his eye during his training alongside Mihawk, but, as the time passed, we came to the conclusion that, probably, he’s keeping it close for his own will, so that the scar isn’t a sign of a possible blindness.
But if he’s really keeping it close for his own will, maybe it means that Zoro’s left eye has some new special power that needs to be used only in particular situations, so, what is this power?
Before answering this question, we’re going to make some preliminary considerations. Let’s go!
In the world of One Piece, the famous swordsmen usually wield swords called “Meitou”, which are some incredibly powerful swords; they’re divided into 3 groups: 12 supreme, 21 great and 50 skillful. Following the events of the story, I came to think that some of these swords are cursed and, in particular, they follow a logic like: “the higher a sword is ranked, the stronger its curse is”.
As far as Cavendish is concerned, for example, during the tournament in Dressrosa, he has always been a normal fighter, but when he has fallen asleep, he awakened his “cursed second personality” called “Hakuba” and, thanks to it, he became able to defeat almost everyone. But why does he have to bear such a curse? I believe that “Hakuba” is the physical manifestation of Durandal’s curse, which is Cavendish’ sword.
Taking into consideration another swordsman such as Law, he’s the owner of a sword named “Kikoku” and, although he has never shown any sign of being possessed by some curse, we know that he lived a tremendous childhood, full of pain; I think that this could be considered as a curse in some ways.
Another character is Shiliew, who’s the captain of Blackbeard Pirates’ 2nd ship; he too has never shown any particular sign of a possible curse and we don’t even know his sword name, but we know that his sword is supposed to increase his desire for blood and killing; I consider this too as a kind of curse.
Coming to the strongest swordsman in the world, Mihawk usually wields Yoru, one of the supreme Meitou; I think that, thanks to the level he reached, he’s aware of the mechanism which the swords follow in order to apply their curses on their owners; the only difference with the others swordsmen is that Mihawk is strong enough to have achieved controlling his curse and its powers. In addition, Cavendish’ Durandal and Mihawk’s Yoru seem to give their owners the same type of eyes.
We know that Zoro owns a cursed sword, named “Sandai Kitetsu”, which he got in Logue Town; in my opinion, because of the curse of Sandai Kitetsu, Zoro has been possessed by 3 demons: we saw the first one when he faced T Bone, the second one when he slashed the marines in Enies Lobby and the third one against Kaku. Of course, being possessed by more than one demon gives him an incredible strength, but also consumes his body and his mind; he’s helped by Kuina’s sword, “Wado Ichimonji”, which is capable of keeping his mind intact.
I think that, during his training alongside Mihawk, the Shichibukai member pushed Zoro’s body so beyond its limits that also Zoro’s mind lost the control; in this situation Mihawk probably used Yoru immense power to seal Zoro’s cursed power in his eye; as long as he keeps it close, that power won’t be freed again.
In conclusion, you can also find some similarities between Zoro and Silvers Rayleigh; I believe that Rayleigh’s sword is cursed too and that also Rayleigh got his curse sealed by someone who slashed his right eye, but the curse was so strong that freed and possessed him completely. Then, as the time passed, Rayleigh became able to control that curse and use it at will and this is the reason for which he’s called “The Dark King”.