What information is available on this female pirate comes from traditional sagas told in Viking society, and put to paper by the Danish historian Saxo Grammaticus.

It is difficult to know how much of Alfhild's story is fact and how much is fiction.  Much of the story we have of her is based upon stories retold by bards in Viking halls.  Best guess is that Alfhild lived during the 9th century A.D.  She was a Danish princess, a daughter of Siward, King of the Goths.  In her minority she was kept under guard of a couple of snakes, until some hero should be able to successfully enter the room.  This is where prince Alf, son of Sigar, king of Denmark, enters the story; he bests the snakes and having done so was to have the hand of the virginal Alfhild, if she would have him.

It is at this point the Alfhild rejects the hero and dons man's attire and begins the life of a rover, which is to say one of Viking rapacity.  She gathered a crew of like minded Valkyria and happening upon a group of rovers lamenting the death of their captain commandeered the vessel and crew as her own.  So in the tradition of Sela, Hetha, Wisna and Webiorg, she went to sea and proved her valor beyond that of other women.

Although she seems to have been, to this point, ignorant of the warrior craft as well as that of seamanship; she and her crew are supposed to have flourished.  According to tradition she and her crew would have sailed about raiding any shipping and settlements that they could find, much of which would have been in and around the Baltic and North Seas.  It was not altogether unknown for women to take up the profession of arms; Vikings were not so narrow minded as Victorians or even nations of today.

By the end of the tale, Alfhild commands a fleet of ships, and it is possible that she contracted her fleet out as mercenaries, or took up true piracy.  She is known to have been a great annoyance to the Danes.  So much so that prince Alf takes to sea to find and end her depredations.  Upon finding her fleet and in true heroic fashion, being outnumbered, he attacks and in the end secures the princess and finally makes her his wife, and sets sail for Denmark.  

A sweet story, but true?  Who knows, it is not doubted that Alfhild existed, the pertinent genealogy bears of that she did.  Was she a pirate or raider/rover?  It is possible, but we do not have anything to prove the case either way.  The stories would have evolved over years, decades and even centuries of retelling to please whichever audience the bard was addressing, so that like the stories of Robin hood, Beowulf and Roland the story would have retained its kernel of truth, but lost its factual content in all but semblance.