You find Chronicles of information documenting the villain's from previous crimes around Arkham Island. they are located everywhere but they are fairly easy to solve. The following guides will help you find any articles you may be missing:
Once you have collected everything and all the stones you will hear from the Riddler one last time. Once he has finished talking head to the Penitentiary and go to the security booth. Head up into the booth and scan the big circle on the floor, the achievement will unlock once you collect this last article.
If your having trouble finding the security booth its in the same room with Clayface and the mysterious man with one arm.