Near the flaming bat you'll find Azrael kneeling down waiting for you. During the first 30% of the CITY OF FEAR story mission, Alfred will inform you about a burning Batman symbol on a rooftop in Gotham and you'll have to investigate this flaming bat. Like most of the secondary missions of Batman Arkham Knight, this can also be unlocked by continuing in the main plot. Batman Arkham Knight : how to unlock the Heir of the Cowl mission With this guide you will find out what Azrael wants from the Dark Knight and get the Angel trophy/achievement in the dark.
Batman Arkham Knight Guide of mission - Heir to the Cowl, A mysterious masked 'hero' is leaving fiery symbols of Batman on the rooftops of Gotham.