Dark Fire is Firebrand's beam hyper combo. When activated Firebrand releases a stream of purple fire from his mouth. Which sweeps up the screen. Firebrand releases the stream at the ground and move his head up where it ends. This move can OTG his opponents and send them up as well with the fire.
The damage it deals depends on how the hyper combo was activated. This move deals the most damage when Firebrand is right next to the opponent or right below him. This ensures that the initial hit sweeps them up with the fire. If this move is activated from a distance its potential damage has been reduced severely since it most likely deal virtually no damage.