Undead are literally the dead that are risen by Necromancers. Depending on the state of the corpse undead can be skeletons, zombies, mummies or ghouls. They will all be mindless and just focused on the task their master has given them. This is also why most undead will not have access to magic at all. They are but an empty shell of the person they used to be. It’s literally the necromancers power willing the lifeless body into moving and often blindly attacking anything and everything that is standing in their way.