The Dread Scroll (Demon Fighter Scroll in the Japanese version) is a promotional item from Fire Emblem: Awakening, and is one of the rewards for completing the DLC chapter Lost Bloodlines 2. Any male character can access the Dread Fighter class by using a Dread Scroll when they're Level 10 or over (or any Level, if in a promoted class).
Using the Dread Scroll does not permanently unlock the Dread Fighter class for a unit. If reclassed out of the Dread Fighter class, another Dread Scroll is required to reclass into it again. Lost Bloodlines 2 will reward the player the Dread Scroll each time it is completed.