Mask of Zero

The Mask of the Revolution and leader of the Japanese terrorist and self proclaimed knight of Justice of Area11/Japan, is an important object within Code Geass, as both a symbol and as a way to conceal Zero's true identity in his fight to take down the Holy Britannian Empire.

About
The Mask itself is nothing very special or unique aside from its design. It is meant to be both a symbol for people to rally behind while also masking the identity of the man beneath it. Lelouch's plan from beginning to end revolved around the donning of this symbolic identity and than passing it down so that the ideals that it represented could be rallied behind.

In order to utilize his Geass without removing the mask, as it requires eye contact, a small chamber of the mask opens to reveal the holder's left eye. It is unknown if one also exists for the right eye as well, though its not impossible that Lelouch could've forseen such a thing and the need for a right eye could've been included given his proclivities towards both pragmatism, expecting the unexpected and dramatics.

History
After obtaining the power of Geass through his contract with CC, Lelouch used it on a tailor to create his getup, the mask included, and debuted it in recognizing Shuzaku Kururugi from his impending faux trial and subsequent execution. During the negotiations with Jeremiah Gottwald, the part that unveiled his eyes was uncovered allowing him to geass the Elite Pilot without exposing his identity and make him "do whatever necessary to allow them to get away or else Orange would be made public" (Orange being something that he had made up in order to sow distrust). Making an impact by this rescue and confessing to being the killer caused unrest and unexpected doubt cementing his immediate legacy the need for additional tropes to quell the Area11 unrest led by Cornelia Li Britannia, as well as a demotion for Jeremiah.

Unlike the Geass the Mask was physical evidence and so Lelouch hid it in a box beneath his bed when not in use. However when Suzaku's cat Arthur took the mask hijinks ensued as Lelouch scrambled to make sure his identity was not exposed.

In the finale of the series Suzaku and Lelouch faked the former's death allowing him to don the mask and make a public execution of Emporer Lelouch.