Blight is the term for the physical corruption of the land by the presence of the Lich King's undead plague. The plague affects all it touches, sterilizing the land, warping its natural inhabitants into bloodthirsty mutants, and twisting the local flora into disgusting mockeries of proper plants. The Eastern Plaguelands (most specifically Plaguewood) and the Dead Scar that runs through Quel'Thalas are perhaps the most startling examples of the long term effects of Blight.This section concerns content exclusive to Wrath of the Lich King.
The lower levels of Zul'Drak are under siege by three necropoles. In addition the Scourge is making use of the blight to turn these areas into sanctuaries for the undead. Their intention is to corrupt the entire zone.This section concerns content exclusive to Warcraft III.
One of WarCraft III's main races, the Undead, can only summon buildings on blighted ground with the exception of the Necropolis and the Haunted Gold Mine. With each finished building, another layer of blight appears around it. Blight is an essential mechanic for the Undead because their creatures regenerate faster while on blighted soil.