Access via Dark Iron Highway, The Hall of Crafting or by jumping down from The Shadowforged City
Incendius!, Location 8.
The area is guarded by mobs of 3 fire elementals. Because they deal insane damage (especially for healers), it is crucial to kill them one after the other. Disable them if possible (Frost Trap, Off-Tank).
Incendius himself is not very hard, just let the tank gain aggro and then blast him away.