So, this raises the question, how does she know what an Avec is and how does she know what the sword does?
She acts as if she's speaking from experience with using the sword, and as if this has all happened before.
If I were to guess, God's plan is basically to have two adventurers fall madly in love, and then use the sword to defeat the demon king. However, he always gets too attached to the saint to let her go, so he has the one she falls in love with die tragically and turn into an undead before he takes her back.
So, another question is raised, who was Soffione in love with? Well, the way she describes it, she was praying to God endlessly to cure the death of her companions but no one responded so she turned her back from God. I think she was in love with one of her companions.