The Black Blight is a magical bio-weapon engineered by the Marcon Alliance. It was used in two capacities: scorched earth and area-denial. The Scorched Earth type was laid behind a retreating army to salt the earth with a virulent plague that killed plants and poisoned the soil. This type would eventually run its course, and in time the land could bloom again.
The second type was used to "lock down" a region to prevent its use by anyone else: either to secure resources or simply to create a geographic boundary. While the blight was not inherently deadly to humans, its secondary effects were quite dangerous. If used over a large enough area, it could make the land impassable.
The blight takes the form of a virulent black mold. It is toxic to humans if ingested, and its spores cause respiratory problems. Any plants infected by it become poisonous, which quickly kills off insect and animal life. The blight comes complete with a kill-switch, and can be "shut off" magically. It requires magic to remain alive, and can be destroyed by an anti-magic aura. When used as an area-denial weapon, the blight can remain dormant indefinitely, activated by water, and powered by a permanent ritual site.
The blight is powered by a 'dual-lock' ritual site, consisting of eight ritual elements representing the four cardinal directions and four anti-elements representing the anti-directions. Because of the combination of normal and dark magics, dual-locked rituals are unusually difficult to disrupt.