Speak with High Priestess MacDonnell at Chillwind Point, Western Plaguelands.
Within the lands of former Lordaeron are eight cauldrons - four here in the west, and four more ensconced in the east. These cauldrons spew poisons into the air, propagating the plague throughout the land for the Scourge.
The necromantic might of Kel'Thuzad is too strong to outright shut down these cauldrons, but we think we might have a way to subvert them to our advantage. My second, High Priestess MacDonnell, will be your point on this assignment; she'll continue the briefing from here.