Revenants

Although it is almost never mentioned, it is common knowledge amongst that military that after any major battle at least a few of the dead will rise again. These revenants remember nothing, are incapable of communication, and attack any living creature on sight. It is often the job of a specialist unit to patrol the edge of a fresh battlefield at night and make sure any revenants that come walking are permanently put down. Unfortunately, since they no longer rely on any of their internal organs, it is hard to kill them with firearms, and most units resort to hacking them down with swords.