Miasmic Malignifier

This is propably where Mortarion stores all his incense fuel from.

The Miasmic Malignifier is Papa Nurgle's personal chimneys to go down every Nurgmas night to eat corpse cookies, drink down some aging pus milk and give every little Nurgling some nice presents.

Overview

On a more serious note, these structures are Death Guard foetid furnaces, which contain all of Nurgle's gunk that are spewed forth as toxic fumes from their chimneys.

This Putrescent Fog makes the Death Guard extra tough and killy by obscuring them in fog within 6" of its range, moreover, this stinks so much like shit that its Noxious Stink deals Heavy 2D6, S 4, AP-1, and D1 to all enemies within 6" of its radius. Even if one is destroyed, the furnace will likely burst in a Putrid Explosion, which, if you roll one D6, on a 4+ it explodes, and any unit within 6" suffers D3 mortal wounds.

Also, you can place it anywhere and seem to grow like a plant. It's Nurgle shenanigans, we ain't explainin shit.