During the period of mass-Christian semi-conversion managed by Olafr Tryggvason, Icelandic saga writers borrowed from Celtic folklore the meme of the super-powered female antagonist. But this was not a stock Nordic character, and so the Icelandic writers turned these Celtic villains into trolls. Usually the mighty troll ladies were slain by the saga hero, though sometimes spared in exchange for treasure, or befriended, as in the case of Brana & Halfdan. Or so Martin Puhvel (1987, McGill) said.
Normal but vengeful females played the boss villain role in Celtic folklore, and usually it took the help of an animal, such as a dog, for the male hero to defeat them. Cats were however associated with the abjected feminine and villainous hags, the cailleach.
As Silvia Federici documented (1998), the crippling fear of women is often the result of instrumental, imperial-cosmopolitan divide-and-conquer interventions imposed upon hinterlands communities. Eleanor Hull (1927) showed that female Celtic supernatural villains like banshees and various evil, watery tarts were degenerate descendants of ancient, mighty war goddesses like Macha and Bodb.