Why Is Zoro's Eye Closed?

Zoro's left eye has glown multiple times throughout the series, most prominently during the Water 7 saga. In fact, Zoro not opening his eye could be him successfully controlling the power and surge of the demon Asura, which he couldn't before.

Is Zoro's eye permanently closed?

Still, nothing has been revealed if Zoro has some special power in his eye due to which he's keeping it closed. All that's been shown till now is that the scar has greatly injured his eye due to which he cannot open it.

Is Zoro blind from one eye?

It was there Oda opened up about his decision to blind Zoro in one eye back in the day, and it seems the mysterious injury was planned from the very beginning of One Piece!