The Dead Scar is a path of Blight that runs through the Kingdom of Quel'Thalas. During the Third War, the armies of the Undead Scourge, led by the death knight Arthas Menethil, Crown Prince of Lordaeron, marched up this path from Lordaeron all the way to the Sunwell Plateau, killing everyone and destroying everything in its path. The Dead Scar practically splits Quel'Thalas in half, all the way through Silvermoon itself.
The undead presence along the Dead Scar is still considerable, even near Silvermoon; though they are fairly weak, their numbers are enough to keep the Silvermoon Ranger Corps busy, fending off the undead marauders. As one travels down the Dead Scar into the Ghostlands, the undead minions become stronger. The strongest undead legion in Quel'Thalas is behind the walls of Deatholme, the Dead Scar's southern terminus — where the traitor Dar'Khan Drathir and his lieutenants command the Scourge efforts in Quel'Thalas.
On occassions, particularly powerful necromancers will gather a large enough force of undead within the Dead Scar to threaten greater Quel'Thalas. Generally, the dissolution of these undead armies are left up to the skilled hand of the Farstriders, but on occasion, the Phoenix Guard must be summoned to put down the threat.