The plague cauldron in Deatholme, overseen by a Deatholme Necromancer.
Plague cauldrons (aka Scourge cauldrons) are giant cauldrons filled with the vile toxins and poisons of the Scourge. Infused with the necromantic sorcery of Kel'Thuzad, these cauldrons are constantly boiling and spewing the Plague of Undeath into the air; corrupting and twisting the land, and empowering the agents of the Scourge. Each of the cauldrons has its own unique mix of toxins, and is guarded by a Cauldron Lord with a retinue of formidable Scourge soldiers. The vast forces that protect each cauldron make these landmarks the second largest concentration of scourge in the area, behind Andorhal. Recently agents in the employ of the Argent Dawn were able to defeat the Cauldron Lords, and gain samples of the deadly toxins, which are still used to this day by the Argent Dawn in a constant battle to try and counteract the deadly concoctions. Heigan the Unclean is the mastermind behind these cauldrons.
Origins
The enchanted cauldrons are most likely from Naxxramas, originally developed by the Lich King, but further researched and perfected by the mortal Kel'Thuzad. In preparation for the siege on the north lands, the Cauldrons were moved to Kel'Thuzad's recent acquisition, the Barov Manor on Caer Darrow (Scholomance) where the Plague of Undeath was perfected within them. Using samples from the cauldrons, Kel'Thuzad began to poison grain shipments exported from Andorhal, thus dooming the populace of Lordaeron to the plague. After Arthas's final "purge" of the region, the cauldrons were ordered to be placed in the various fields around Andorhal by Araj the Summoner to help support the scourge forces in the area. Whether this was a tactical decision (cleared, level ground easily protected), or an ironic monument to the beginnings of the Scourge (the cauldrons sitting in the very fields that produced the corrupted grain), is unknown.
In-Game Cauldron Locations
Western Plaguelands
Eastern Plaguelands
- Plaguewood (two)
- The Fungal Vale
- The Noxious Glade
- Ruins of the Scarlet Enclave (in what used to be the Havenshire Farms)
Ghostlands
- Outside the Tower of the Damned, Deatholme
Mount Hyjal
- Inside Scourge area in the instance Battle for Mount Hyjal.
Scarlet Enclave 
Building a Cauldron
Death knights are tasked with aiding Noth the Plaguebringer in the creation of a Scourge cauldron at the Havenshire Farms [56.1, 52.1]. It is involved in the following quests:
[55] The Plaguebringer's Request- [55] Noth's Special Brew
- [55] More Skulls For Brew
The cauldron is no longer available after the death knight has completed the quest 
[55] The Light of Dawn, although it can still be seen in the ruined farm afterwards.