The Hellfire Swarm is the Video variation of Delsin's Karma Bomb moves. It is the last Video power you gain in 'Zero to Hero'.
The sky grows dark and angels or demons (depending on your Karma) are summoned and they rush past Delsin into the air. If you have good Karma, the angels will divebomb down in a line and obliterate anything in range. If you have evil Karma, the demons will go divebomb towards the enemies instead.
Some enemies will try to shoot your Angels or Demons while they fly upwards.