The undead are a broad subject in of themselves, but the most common association with the term is reanimated corpses, colloquially known as zombies, or a "Frankenstein," in reference to the famous novel of the same name. The context behind the reanimated can be varied, but in all cases, they are individuals who have been brought back to life by an individual to serve some purpose.
More information about the reanimated will be provided in the future.