A draugr or draug (original Old Norse plural draugar, as used here, not draugrs), or draugen (Nor., Swe. and Dan., meaning "the draug"), also known as aptrgangr (lit. "after-goer," or "one who walks after death") is an undead creature from Norse mythology. The original Norse meaning of the word is ghost, and in older literature one will find clear distinctions between sea-draug and land-draug. Draugar were believed to live in the graves of dead Vikings, being the body of the dead. Views differ on whether the personality and soul of the dead person lingers in the draugr. As the graves of important men often contained a good amount of wealth, the draugr jealously guards his treasures, even after death.
Draugar possess superhuman strength, can increase their size at will and carry the unmistakable stench of decay. The draugr's body, however does not decay and this odor was most likely due to the draugr being a host for diseases. The draugr's ability to increase its size also increased its weight as it did so, and the body of the draugr was described as being extremely heavy. It is possibly that the draugr could also be heavy without increasing its size, as this could help explain its immense strength, but the strength of the draugr could also be independent of its size or weight. The weight of the draugr may also only increase as size does. Thorolf of Eyrbyggja Saga was "uncorrupted, and with an ugly look about him... swollen to the size of an ox," and his body could not be raised without levers, it was so heavy. They are also noted for the ability to rise from the grave as wisps of smoke, possibly meaning they also have the ability to turn into smoke at will, although it is much more likely that this is an interperatation of their ability to "swim" through solid rock, which would be useful as a means of exiting their graves. Whether or not they can become intangible to other materials is not certain. In folklore the draugar slay their victims through various methods including crushing them with their enlarged forms, devouring their flesh, devouring them whole in their enlarged froms, inderectly killing them by driving them mad, and drinking their blood. Animals feeding near the grave of a draugr are often driven mad by the creature's influence.[ They may also die from being driven mad. Thorolf of the Eyrbyggja saga, for example, caused birds that flew over his howe to drop dead. Draugr are also noted as being able to drive living people insane. This may have been a method of the draugr to create more of his kind, similar to a vampire. The eyes of the draugr may be connected to this as their eyes are noted as being terrifying to look at, causing intense fear in even the bravest heroes. The power of the draugr's eyes, however, are never fully explained.
The draugr's victims were not limited to trespassers in its howe. The roaming ghosts decimated livestock by running the animals to death while either riding them or pursuing them in some hideous, half-flayed form. Shepherd, whose duties to their flocks left them out of doors at night time, were also particular targets for the hunger and hatred of the undead:
"... the oxen which had been used to haul Thorolf's body were ridden to death by demons, and every single beast that came near his grave went raving mad and howled itself to death. The shepherd at Hvamm often came racing home with Thorolf after him. One day that autumn neither sheep nor shepherd came back to the farm."
Draugr are noted for having numerous magical abilities, referred to as "trollskap" resembling those of living witches and wizards, such as shape-changing, controlling the weather, and seeing into the future. Among the creatures that a draugr may turn into are a seal, a great flayed bull, a grey horse with no ears or tail and a broken back, and a cat that would sit upon a sleeper's chest and grow steadily heavier until the victim suffocated The draugr Thrain shape-shifted into a "cat-like creature" (kattakyn) in Hromundar saga Greipssonar:
"Then Thrain turned himself into a troll, and the barrow was filled with a horrible stench; and he stuck his claws into the back of Hromund's neck, tearing the flesh from his bones..."
More on this unexplained mystery at http://theunexplainedmysteries.com/Draugr.html