Story-related; unlocks at the end of the final DLC story mission, ‘The Sound of Violence’.

The Dirge boss fight requires you to kill hordes of zombies near the Orgone balls to power them up, something you have done many times in the DLC up to this point. Then bait the Dirge close to the main speaker and throw a powered up Orgone ball at it to make it vulnerable. You can then directly attack the boss. You'll need to do this for three phases to defeat it. A full video walkthrough of the SoLA DLC story, including the Dirge final boss fight can be found HERE!