Speak with Shadow Priestess Vandis at the Bulwark, Western Plaguelands.
Within the lands of former Lordaeron are eight cauldrons - four in the west, and four more entrenched 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, Shadow Priestess Vandis, will be your point on this assignment; she'll continue the briefing from here.