The final result of a person suffering from the Border Disease, which ends turning the infected ones into mindless zombies that can only repeat a single task endlessly, the most common of these being killing or other destructive tasks.
This is the second true boss battle you will have during the game, and while they are pretty much improved versions of the Archia Soldiers and Great Fang Soldiers you will be meeting shortly later, these enemies aren't to be toyed around with: their stats are pretty high for this point in the game, and if you get careless, they will effortlessly kill your Reyvateil and all of your party members. Therefore, make sure to keep an eye on your HP all the time, and try to get your Reyvateil to her Lv. 3 Purge as fast as you can to destroy them, as their high resistance to magic would render any Song Magic from a Lv. 1 or Lv. 2 Purge useless.