The Conflagration

The Conflagration can be found at Corp'rethar: The Horror Gate in Icecrown. It is the site where the Lich King's elite soldiers have formed up to defend the Citadel and it seems to thank its name to the large blue flames in front of the gate.

Thassarian or Koltira Deathweaver send the player to weaken the Lich King's forces by dissolving the corpses of the Skeletal Reavers thus making it easier for the upcoming main assault to reach the last line of defense: Corp'rethar.