The Ashen Kingdom
Wars are in some ways self-defeating. Combat being what it is, battalions never manage to recover all their fallen comrades. Dead or alive, most who spend a night in the Forest walk back out changed.
The undead are reborn humans. Some are mindless and simply hunt their former friends. Worse is when the creature rises with a degree of predatory cunning. There are instances of undead being so clever and convincing that they simply return to their homes within the palisade. Such creatures often have a variety of terrible powers and can wreck terrible destruction in a single night.
Some undead return as incorporeal ghosts. While most undead can be slain with applewood, silver, fire or iron, ghosts can only be undone by recovering the body and burning it to ashes.