Blight is the term for the physical and spiritual corruption of land caused by necromancy. While it is similar to scorched earth, blighted land does not regenerate over time. Particularly potent corruption impacts living things that feed off the land or live within it. Blight also causes fallen corpses to reanimate continuously. These corpses are bound to the original will of the necromancer that created the blight, even if that necromancer is no longer living. In Quel'Thalas, blight has been extremely detrimental found within a large swath of land known as the Dead Scar that cuts the country in half making trade and travel dangerous.
Undead warbands often form in heavily blighted areas to ward away natural regeneration. These warbands can often be manipulated or bound to the will of a more powerful necromancer.
There has been some successes with removing blight by druidic magics, however this process is slow and takes tremendous resources.