Undeath is a condition that can occur to any sentient creature. After physical death of the body (current guidelines by the Imperial Medical College specify that physical death occurs after heartbeat and breathing has ceased for at least 3 minutes), the soul remains attached to the corpse for a period of 2 days. After this period, the soul detaches and moves on, though no one knows where.
During this two day window, the energy that is called the soul remains vulnerable to manipulation. Only human Mages have shown skill at this art. With the proper rituals, a Mage can rebind the soul the body permanently, and animate the body into a semblance of life.
There have been reports of naturally occurring cases of undeath, primarily when someone who has died and left something important behind. In these cases, it seems that a strong will can, at least temporarily, keep the soul attached to the body and animate the body. And sometimes it seems random. No one is certain why the rare corpse rises again as a undead.
The undead have several characteristics. They are not vulnerable to mundane diseases or poisons. There is no living biology for these agents to feed on or to affect. Breathing is no longer necessary, nor is eating. All movement and activity is fueled by necromantic energy. The undead are physically superior to the living. They can push their bodies beyond normal limits, and regenerate their bodies extremely quickly. They no longer need sleep, and are able to work tirelessly around the clock. With enough time and practice, an undead can even learn to manipulate the dark energy that animates him to achieve something resembling magical effects.
With all these advantages, however, come many disadvantages. The first is dependence upon vital energy. Their own bodies have no way of producing energy, so it must be obtained from some other source. If the undead was created by the Empire, then the Black Flame Order maintains them through amulets infused with vital energy that the undead can draw upon for sustenance. For the rogue undead, this vital energy must be obtained from the living. Animals cannot meet this need, only sentient creatures produce enough to satisfy the undead.
The easiest way to obtain it is to the eat the flesh of the living. This has the predictable side-effect of often killing the victim. A safer way is through blood, as long as the undead does not take too much. The more clever and intelligent of the undead often steal a little blood from multiple people in a village over time, hiding their predations yet keeping them sustained. Most rogue undead will fiercely defend their territories of humans. Brain and spinal tissue are rich in vital energy, and are where the rabid undead like to feed from.
The intelligence of an undead can vary widely, from little more than a rabid dog to a level of intelligence equal to what they possessed in life. In the case of undead created by ritual magic, the resulting level of intelligence is largely dependent upon the skill of the mage reanimating them. The Empire, however, forbids the creation of fully-intelligent undead, and limits them to enough intelligence to be able to understand and follow orders.
In the cast of spontaneous undead, it seems almost random. However, the more strong-willed a person was in life, the more likely it is that they will possess a higher level of intelligence in their undead state. Given time, wild undead can increase their power with age. If they constantly feed more than is required to sustain their existence, their intelligence will increase until it is equal to what they had in life. In some cases, it can even go beyond that. However, if they go without sufficient feedings for a long enough period of time, their intelligence will begin to decrease.