Firestorm is a fire attack used by Diablo which is like a more powerful version of the Druid's Firestorm Skill. It is also available for player characters, but only as a proc, from the Hellfire Torch.
Diablo typically casts Firestorms at somewhat close range, even though it will travel much farther than the deadly Red Lightning Hose. It is fast moving and tends to cover a wide path with random movement, making it a little difficult to outmaneuver, but quickly running perpendicular to its direction of carnage will often prove successful. Minions, however, are easily caught by the attack and killed instantly, making their usefulness, even as decoys, rather short-lived. It is advisable for Necromancers to draw some of the monsters which appear when the Seals are activated near this area before killing them, to ensure a supply of corpses nearby, for the use of Corpse Explosion.