They are the different kind of the undead because of their appearance which is shown with pitch-blike eyes on their faces and like any other zombies in fiction and real life, they can be reanimated through magic.
The undead were unleashed upon a small American farmland and soon the whole modern world if left unopposed by Mathias as he did in the past before. He intended to use an amulet to increase his sorcery and permantly summon and control the dead during daylight the he did in night. His deadly plan of conquest and his legions of undead slaves were stopped by the young swordsman Jacob.
But at a terrible cost: during Jacob's final fight with Mathias and his undead minions, Sarah who was protected by Jacob from him, was killed when her mother after being bitten and infected by the undead, became one herself and murdered her own daughter.