A new evolution of the Plague and the Blight.
Too many modular animates have been disposed of. The smoke of the fires, the chemical smog of the alchemists, the dirty fogs of London and the miasma of the undead have formed a new entity.
A malevolent black cloud.
It drifts low to the ground and can touch it again to form very thick fogs. It can be controlled by Zombie Lords and will accompany them as they attack, masking them from ranged weapons.
Incidence of Plague infection increase whenever the cloud is present.
At some stage in the future, authorities fear that the cloud will rain - destroying itself but infecting whatever watersource it ends up draining into.