You must find and destroy all 20 of Black Mask's masks. Unlike treasure chests, these are not marked on your mini map, and most are well hidden in the background and foreground. They will however shine green when analyzing in detective mode. You must use your Batarang to destroy them.
The following video shows all 20 locations. Also, they are not cumulative and must all be found on one play through.
Video credit goes to YouTube user "Gaming Since Gaming"