What happened to Roronoa Zoro’s left eye?

Mihawk’s freaky eyes are part of the key to why he is such a great swordsman. As Shanks has given his arm to Luffy for the new generation, Mihawk has given his eye to Zoro.

Zoro will get into some insane fight where it seems impossible he can win. Right when he is on the brink of defeat, he will smile or maybe even laugh and completely catch his opponent off guard. “You’re pretty strong, I guess I’ll have to use my strong eye.” He opens the left eye and clearly has the same eye as Mihawk. We then find out he is twice as powerful, and he blows the seemigly insurmountable opponent out of the water.

As to why he didn’t use the eye before this. His prior opponents may have been too weak and he wanted some competition. Alternatively or additionally, the eye may be some kind of burden. It could be demonic or he could despise it since he is prideful and wanted to become the greatest swordsman using only his own power. Taking and needing the eye was a devastating realization during the time skip which now compromises his honor.