Draugr

Only the most malevolent of people could become one of these vile creatures. Any extremely greedy or evil person may become a draugr after death. They will then protect their place or treasure from anyone who comes near. These undead creatures are extremely strong! They are so powerful that no normal weapon can harm them permanently.
They are restless undead and cannot lie down. Therefore, when they are first seen they will either be sitting or standing. They may be seated upon a throne, chair, or upright within their own grave. Draugr have the ability to increase their size to five times their mortal mass. Their strength and willpower are beyond those of mortals. While their skills are only those they had while alive, the other advantages they have more than make up for any lack of skills they may have had. They never tire. And they may appear as gigantic, bloated, disgusting corpses; half flayed demons or undead; or merely as shriveled bodies - as they will.
A draugr has several magical abilities, as well. They can control the weather in their immediate area. They can shape shift into several creatures, including a skinless bull, a seal, a great maimed horse with a broken back and no ears or tail, or a cat. In their feline form they are known to sneak into the home of those who wronged them as mortals - or stole from them after death - and sit upon the sleeping person's chest gradually growing heavier and heavier until suffocating the person.
The extremely powerful draugr may even turn itself into a troll with giant claws and tough skin and a horrible stench.
Their other magical abilities include:

• the ability to visit people's dream and even leave behind small tokens to show that the dream was real
• to curse people with weakness or disease
• to create temporary darkness during the day
• curse, even kill, people with bad luck

Because they are mostly immune to weapons they must be wrestled back to their grave before they can be defeated - and only there do they become vulnerable to any weapons, which must be of a magical nature. To destroy a draugr one must cut its head off, throw the head in the sea or bury it deep in the earth, burn the body and scatter its ashes in the sea.
A less powerful version of the draugr is the haugbui.
An even more powerful and dangerous version of draugr is the bholgar.