The Army Beyond Death
The capabilities of the undead created depend on the soul used. The weakest souls, such as of very small animals, will not create a usable undead. The souls of regular animals create low level undead, with only rudimentary intelligence. It is the souls of the intelligent races which create undead with intelligence, capable of strategy, communication and so forth.
Further, only the soul of a magic user will create a magic using undead. The most powerful undead tend to come from either the souls of powerful individuals, or of great beasts, creatures and monsters such as dragons, beholders and powerful demons.
There is no evidence to suggest that the undead retain any of their past lives. The undead do not seem able to access the knowledge of the dead, nor have any displayed a recognisable personality of a now dead person. There have been times when attacking undead have carried the faces of the deceased, but it is thought that this is a trick of psychological warfare, rather than any indication that the individual’s essence lives on after death.
Upon being slain, the soul within the undead dissipates, finally being allowed to pass on to the afterlife.