The Plague of Nightmares was a powerful spell which Distreyd Thanadar XII, High Cleric of Mardük, was continuously casting over the Land of the Living between 1000 AE and 1005 AE. Distreyd channeled his Innate Ability through the spell and could focus it throughout the globe, on individual nations or even specific areas within said nations at a time thanks to leeching magic from a large shard of Krystallopyr. This leeching boosted his powers to incredible levels albeit with the cost of making his body wither rapidly over the years he kept casting the spell.
The plague made everyone in the spell's target area, friend and foe alike, have horrifying nightmares every night, often preventing people from sleeping for extended periods of time and thus making them more fatigued as a result, which allowed the army of the Yamato Empire to invade the areas Distreyd had targeted earlier with relative ease.
The spell was finally nullified when the Grand Alliance inserted the Cricket Ball of Magic into an ancient reactor core underneath Lutherin. The act sent a magical shockwave which shattered Distreyd's Krystallopyr shard and weakened him as a result, but also led to the Reactor Core Explosion which destroyed Lutherin and led to the eventual rise of Alent in its place. The severely weakened Distreyd was assassinated by Jemuel soon after, and his base Vulpengaard Keep was looted during the First Battle of Vulpengaard Keep.