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'.
Delsin sticks his arms forward as the sky grows dark. Shortly afterward, an Angel/Demon (Angel for Good Karma, Demon for Evil Karma) will fly past him into the air, then another, then a massive group of them. Once a large enough group of Angels/Demons have formed, Delsin raises his arms and swiftly brings them down, signaling the flying army to divebomb a large area in front of Delsin, subduing enemies and leaving civilians uninjured (Good Karma), or obliterating everyone in sight (Evil Karma).
Some enemies will try to shoot your Angels or Demons while they fly upwards.
On his fight with Augustine, Delsin summons them a lot faster compared to other times that he uses them.