Kroenen is a draugr
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The Draugar are undead monsters from Norse mythology, brought back from the dead by the sheer force of will, usually to take revenge or because they're just that evil. They are descriped to retain human-like intelligence, look like corpses, and are immune to weapons. .Kroenen died in the movie's beginning when he was impaled,and rose from the dead to take vengeance on professor Broom, and can shrug off being shot at because of being a draugr.
The draugar, though, are descriped to have several other attributes Kroenen lacks.
