Where are the voices in Yngvild's head coming from?

Yngvild grew up on the small island of Straven, with her foster brother Noren, and a small group of adults that crew a fishing boat and look after a flock of sheep. It's a quiet backwater in the Western Isles, and apart from the odd visit by raiders, pretty safe. Until the King's Ship Seagull comes visiting, to keep an eye on the mysterious visitor from afar that has turned up unannounced.

Nicknamed 'Yngvild the Fierce' because at the age of 14 she scared off raiders by bouncing a practice arrow off the helmet of their leader, twice. Yngvild and Noren go to the capital, Kronstadt, with Old Bjorn, the man she thought was her father. Helga Trollslayer, Yngvild's long-lost mother, passes Yngvild the sword known as Jafnadr, an heirloom from her dead father. Yngvild, Noren, and others, are heroes descended from the gods. On arrival at Krondstadt they're dropped into the murky world of heroes, and the trouble begins.

Now a young woman with voices in her head, which she thinks are from the legendary sword she's just been given, or maybe from the silver talisman she wears round her neck. Yngvild sets out with Noren to find out who killed her father, and seek justice...