Demons
In "Dragon Age," undead beings are created when a dead body is possessed by a demon. In the game there are rage demons, pride demons, sloth demons, desire demons and hunger demons. Different demons can affect the corpse in different ways -- a skeleton possessed by a lesser hunger demon may become a fanged skeleton, while a more powerful demon might create a revenant.
Corpses
A corpse is a dead body in a less severe state of decomposition than a skeleton. Demons can possess these bodies and reanimate them. If the demon is a rage demon, the corpse will simply go berserk and attack everything around it. Hunger demons produce devouring corpses that can drain life from you and your party; sloth demons produce shambling corpses that can slow their enemies or put them to sleep. They are immune to Nature damage and resistant to Cold, although they can still be frozen. Corpses are immune to the Sleep effect.
Skeletons
Sometimes, lesser demons seeking a way into the living world find themselves trapped in a skeleton. Higher-level demons might become revenants or arcane horrors, but lesser demons immediately lose their ability to reason, attacking any form of life they encounter without regard for their own safety. Possession of a skeleton by a hunger demon will result in a life-draining fanged skeleton; possession by a sloth demon results in a shambling skeleton, which can slow down your party or cause them to sleep. All skeletons are immune to Nature damage, Cold damage and Blood Mage spells. They are also immune to the Charm and Sleep Effects.
Revenants
Revenants are the result of a powerful demon of desire or pride possessing a corpse. They are more powerful than skeletons or corpses but have similar immunities. They typically have armor and weapons; some also cast spells. Revenants can't be harmed by Nature or Cold damage. Additionally, they are immune to the Charm, Fear, Knockdown, Sleep, Slip and Stun effects.
Arcane Horror
When a pride demon takes over the corpse of a dead mage, an arcane horror is created. These creatures are something like revenants, but also possess the magical abilities that the human had in life. Arcane horrors can be killed fairly quickly but do significant amounts of damage, wielding spells that can devastate your party. They are immune to Nature damage and Cold damage; however, it's still worth casting spells that freeze the subject in place, such as Cone of Cold. Arcane horrors are immune to the Charm, Confusion, Sleep, Slip and Stun Effects.