I seriously think that "Demon Sword" and "Demon Stone" are mistranslated and is quite fairly common mistranslation in these types of story. I haven't checked the raws but I can assume the japanese word for "Demon Sword" is "Maken", the "Ma" can be either translated to "Magic" or "Demon". The correct translation should be "Magic Sword" because there's no demonic aspect in Len's magic. Especially when Lycia apparently has a "Demon Stone" on her which is stupid and probably should've been "Magic Stone" cuz it makes more sense.