The War of the Mark was a terrible and bloody conflict which changed the face of Khorvaire, solidifying the dragonmarked houses' position today. The war began about five hundred years before the establishment of Galifar, when the dragonmarked houses launched an inquisition to put an end to the proliferation of aberrant and mixed dragonmarks.
The houses had the advantage of greater numbers, but the aberrant marks of the time carried tremendous destructive power. In the third year of the war, Lord Halas Tarkanan began organizing his aberrant kindred into a proper military force to oppose the dragonmarked houses. Tarkanan, known as "The Earthshaker", carried an aberrant mark that gave him great control over the elemental forces of earth and fire.
However, Tarkanan's main contribution to the War of the Mark was his brilliance in tactics and strategy. His consort, the Lady of Plague, was an enigmatic woman whose dragonmark power was never specified, but her title and the end of the war suggests that it allowed her to call upon a multitude of small creatures and deadly plagues.
Although Tarkanan's efforts extended the conflict, the aberrants fell in the end. Tarkanan himself was trapped in the siege of Sharn, which had been his stronghold and base of operations. When it became clear that the aberrants were doomed, he and the Lady of Plague unleashed the full power of their marks - power sufficient to destroy the city. Terrible quakes caused the collapse of parts of the city, and rivers of lava flowed up from the fiery lakes below. Masses of pests and horrible plagues ravaged invaders and defenders alike. Some claim that the Lady of Plague's death-curse still lingers in the depths of Sharn.
The War of the Mark ultimately established the dragonmarked families that hold power today, and it laid the foundation for the prominent role of both House Cannith and House Deneith, which played central roles in the Last War. It also led to the foundation of the Twelve as an avenue for the houses to cooperate toward common aims.