So Nintendo just posted a Book V Movie that looks like it could have something happen in the Mid Chapters of Book V that was just announced today. I'm kinda speechless watching in this movie, but it did gave me some ideas what happens in Book V.
Eitri had launched her magic gear canon at Fáfnir that turned him into a Beast that is similar to like Three Houses did with the Crests Users can turn anyone into Beasts.
So I don't exactly know if this is suppose to be Eitri helping Fáfnir or Eitri was manipulating Fáfnir this entire time. I don't know if this really caused Fáfnir turned into evil based upon what Reginn mentioned to the Order of Heroes? I suppose to think Eitri was the true main antagonist in Book V, not Fáfnir. (*gasp*) I'm still kinda wondered why Reginn isn't using her Armor in the movie unless if something bad happened to her armor in Book V, but she does have nice jumping reflexes in the movie? But at the end that it looks like Reginn is pulling the Alm stabbing to the Head at Fáfnir and Fáfnir might going to be killed by her own foster sister, Reginn.