Firestorm

Firestorm is triggered when you enter the Fire Arena in level 2 of the Plague Tunnels in Westmarch Commons in Act V. You are then trapped in the arena where you have to survive three waves of Reaper attacks. Firestorm is a dangerous event because on top of the Reapers, four pillars spin, almost continuously, deadly flames at you. Once the three waves of Revenant Soldiers and Revenant Archers are defeated, you still need to kill Peleil, a Superunique Death Maiden. This is the most dangerous part of the event because Reapers are constantly spawning and the flame pillars keep firing.
Tip: the Western entrance of the Fire Arena does not lock during the event. Exit through it if your life decreases to worrying levels. Peleil will not follow and the fire pillars will not reach you.

Trapped in the Fire Arena (v2.0.6)

The last stage of the event where maximal damage occurs (v2.0.6)