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[Folklore] A Nordic variety of the Icelandic draugr, draugs are most often identified with the spirits of mariners drowned at sea (as for example in Jonas Lie’s Elias and the Draug). Having been denied proper burial themselves, they haunt the shores of Norway to bring doom upon any mariner who sees them. Conversely, they are only visible to their future victims. Draugs are said to have seaweeds for heads and to sail around in half a boat. Some accounts portray them as shapeshifters who take on the appearance of stones in the shoreline. When a mariner treads upon such a stone he faces certain death, unless he would first spit on it. The term can also be used more generally to describe any type of revenant in Nordic folklore.

See more at Wikipedia.org...

[Folklore] A Nordic variety of the Icelandic draugr, draugs are most often identified with the spirits of mariners drowned at sea (as for example in Jonas Lie’s Elias and the Draug). Having been denied proper burial themselves, they haunt the shores of Norway to bring doom upon any mariner who sees them. Conversely, they are only visible to their future victims. Draugs are said to have seaweeds for heads and to sail around in half a boat. Some accounts portray them as shapeshifters who take on the appearance of stones in the shoreline. When a mariner treads upon such a stone he faces certain death, unless he would first spit on it. The term can also be used more generally to describe any type of revenant in Nordic folklore.