The Attack of the Dead Men occurred on August 6th, 1915, after the German army launched several gas attacks on the fortress, killing all but roughly 60-100 men from the defending force. In a last-ditch effort to send the Germans back the men inside charged out of the fortress, many coughing up blood. This sent a wave of fear through the Germans mostly due to their appearance, that being of dead men charging, covered in rags soaked in blood, guts, and vomit.
I do not own the song itself or any of the content displayed in the video. Thank you to the contributors in Genius who helped with the description of the song, and its summary.