The first visible signs of a blight are areas of land seemingly dying, looking like burned places, white as ash. These areas contain sickly gray vegetation and ground that feels "cold"; the very air within them is described as being full of despair. As explained by Magnus Bane, these blighted areas are scars left by the dark magic used for the spell. Serving as doorways, the blight could be used to easily let demons through.
The more the blight spreads, warlocks around the world will slowly weaken and become ill, affecting first the oldest and the most powerful warlocks and those that use magic often. Eventually, the affected warlocks would turn into demons.
The demons came through the doorways first and destroyed civilizations, slaughtering humans who were unable to fight back without Sebastian's protection. The warlock population slowly became ill and was eventually depleted or turned into demons, save for the immune Last Warlock. Finally, the Nephilim became severely weakened, their weapons no longer usable and their runes eventually faded, forcing many either into submission to Sebastian or into hiding, secret rebellion.
After the establishment of the Cold Peace, the Unseelie King employed Malcolm Fade's help and was able to open a Portal to Thule, allowing him to spread the blight in their world. Also as part of his plans, the King primed the half-faerie, half-Shadowhunter with demon blood boy Ash Morgenstern with faerie magic and spells from the Black Volume of the Dead, hoping to use him to easily spread the blight even without the Portal.
He first unleashed the blight on portions of the Unseelie Land. At one point, the Cohort entered into an alliance with the Unseelie King and helped spread the blight in Idris, with the goal of simply making the Clave fear its effects, making them easier to sway for their plans.
In September 2012, a group of Shadowhunters from the Los Angeles Institute entered the King's land on a rescue mission and saw many blighted areas. Around the same time, the blight in Brocelind Forest was also discovered, prompting an investigation.
The first person to realize that the blight was responsible for the widespread warlock sickness was Cristina Rosales; later, Emma Carstairs and Julian Blackthorn were able to confirm this because of their time in Thule. Upon their return from Thule, armed with the knowledge from that world, they were able to thwart the effects of the blight with water from Lake Lyn. Though the land would always be scarred, its dark magic was nullified.
- The blight does not affect runes worn by Clary Fairchild and Jace Herondale, their significant amount of angel blood making them more resistant to the blight's effects. In addition, Tessa Gray (in both worlds) was immune to the warlock sickness dealt by the blight, believed to be due to her Shadowhunter blood.