Norse mythology is full of dragons, from Nidhogg beneath the world ash, to Fafnir who was killed by Sigurd, to Grendel who was killed by Beowulf. Dragons were associated with water, and often lived in seas, misty lakes, storms or fogs. Sometimes they lived deep in underground caverns. The Norse people adopted the totem of the dragon to protect many things, including the prows of their ships and names of their towns.
The description of the upgrade mentions Grendel being killed by Beowulf, however the dragon Beowulf killed in the titular epic was a different nameless creature. Whether or not Grendel is also a dragon is still the subject of scholarly debate.