Draugar are extremely powerful undead and have little to no weaknesses. They are immune to all sorts of conventional weapons. A Draugr must be wrestled into its mound by force, but even then may arise again. The only way to ensure a draugr doesn't come back to a living form is to sever the head from the neck, burn the body and dump the ashes into the sea. For this reason, many Norse warriors were buried at sea on a ship which was burned while sailing forth so their spirits could not come back. The mound would then be opened to "purifying" sunlight.
A Tale of Draugr
Several hundred years later, with Viking society on the brink of collapse, a raiding party decided to raid the mound for the gold within. Lowering down their bravest warrior to the depths of the pit, they fled in terror as Asmund was raised out. He told them that Aswid had risen as a Draugr and came back. He devoured his horse, and his hound before turning on Asmund. Asmund showed them the scars he had sustained at his old friend's hands. They had battled for centuries, Asmund holding him off with his sword, but the distraction of the warrior's arrival meant that Asmund could finally slay Aswid. Tale finished, he collapsed and was reburied by the raiding party, who left the treasures in the mound. Aswid they took out, hacked apart, burnt and scattered his ashes.
The more recent creatures are associated with the sea rather than land. These undead beings were once people who have drowned at sea and have risen from the deep being composed entirely of seaweed. Some have described them as being headless fisherman sailing half a boat or in the form of a living corpse.
To prevent the return of dead Vikings, scissors were placed on their chests along with bits of twigs hidden in their clothing. Their big toes were tied together and needles were driven through their feet to keep them from walking again once dead.
To prevent the dead from rising people would drag a dead body feet first surrounded by a thick crowd so that the dead corpse did not know where it was going. Once placed in a coffin a special door was bolted on to prevent a return visit. This tradition of burying started in Denmark and spread across Scandinavian Europe.