Not geography but an infection, coursing through the landscape. A patch of plague, where parasite-trees claw the sky atop tumour-hills. Infected valleys weep the earth's pus. Yet the Phagelands do not stagnate - competition is rife amongst the many diseases and disorders present, and therein lies it's deadly lure. The huge, complex hierarchies of infections and parasites have led to twisted cures which literally eat other diseases, leaving the host comparatively unharmed - after all, nothing normal lives in the Phagelands.
The plaguescape does have its migratory inhabitants - trains of lepers accompanied by a litany of death-rattles, those driven to desperation by biological ruin hunting across the dying land for the cure to what ails. Diseases are adaptable however, and take their toll on the desperate as they change to infect their new hosts.
The Phagelands move, drifting across the landscape. Though the size varies, it is always at least a days travel wide. Animals flee and plants die as it blindly slides across the lands, unperturbed by obstacles. Cities have been devastated simply through loss of cropland, let alone the nightmarish infestation of direct contact. It has lurked beneath the sea for centuries before, eventually emerging onto land once more, beset with new infections and maladies.
Those who have the misfortune to dwell long in drifting disaster speak in hushed whispers of the Walking Plagues, who have developed an alien intelligence and stalk their dying environs. No healthy human has seen one, and can you trust one who chooses to live in such a place?