Unlike undead, these creatures are primarily free-willed. Although the wizards who create them do maintain a modicum of control, this control is more in the form of a grateful servant rather than blind obedience.
The Shadow Knights are typically reported as mounted, either on great warhorses (possibly which have been subject to the same dark magics) or occasionally on Nightmares. They are always well armed and armored - typically with the enchanted weapons which they carried in life.
Habitat
It is believed that most of those who become Shadow Knights do so out of a
desire to continue their work. One legend tells of the great Dwarven
warrior Dugel Drayspear who rallied his forces at the siege of Ironhold and
defeated the forces of Goblinkind. During the siege, Dugels family all
perished. He became obsessed with the extermination of Goblinkind. Late in
life, it is said that he made a bargain with a wizard to give him
immortality so that he could continue to hunt Goblinkind. They say that
even to this day, he roams the foothills near Ironhold.
It seems that most Shadow Knights have some purpose to which they are devoted. They are almost always solitary in nature. The one exception, if it is indeed true, are the rumors that speak of the Fragment Suliden (Fragment of the Dead). It is said that well over twenty Shadow Knights roam the island - possibly protecting the resting place of former companions or employers.
While Shadow Knights, in and of themselves are apparently not inherently evil, it seems that the magics which create them are. Having said this, I would not say that Shadow Knights are good or even friendly. It has been said that they have helped people in times of need, but at best I would classify them as amoral, with their life's ambition clouding all actions.
Unusual Aspects
Not only do the Shadow Knights retain their fighting skills, but the magic
empowers them with other skills and attributes. It is not known if all
Shadow Knights share the same set of abilities, or if each is unique. What
is known is that all Shadow Knights have the ability to use or manipulate
the fabric of shadows. Shadow Knights can never fully be exposed to
sunlight, at least part of them must always be in shadow - no matter how
small. Whenever they are totally covered by shadow, they are capable of
actually moving from place to place, regardless of distance - by moving
through the shadow realm. As they require shadows, in areas of complete
darkness, Shadow Knights can not exist.
If ever exposed to total light or total darkness, Shadow Knights fade away. This does not kill them, but sends them to the realm of shadow for a period, believed to be at least three years.
In addition to the ability to manipulate shadow, the following has been reported as powers of certain Shadow Knights. Again, I have not yet been able to determine if all Shadow Knights share these powers, or if each is unique. Some are said to have: controlled hoofed animals, walk on wind, telekinesis, telepathy, resist fire, impervious to normal weapons.