you'll have to look up cheat mode but there's a txt file in the game folder that says cheat mode = 0 or something and you just have to change it to 1. then when you open inventory all the items in the game will be available on the left side for you to transfer to your inventory.
i'm not 100% sure about legit. i've only spoused a couple characters. its a battanian sword so one of those nobles is probably going to have it as their civilian weapon. i think corein (caladog's daughter) might actually have it.
alternatively you can smith it, "engraved backsword blade" is a tier V one handed sword piece. and theres a lot of guards and pommels that look like viking swords, gold ones, silver ones, bronze ones
smithing is op right now. all you have to do to level it and get all the recipes is forge 2h swords by buying a bunch of throwing daggers, maybe a couple ores at the start. make the highest damage stuff you can make, you'll get harpoon recipes or something which are worth like 50 levels each. and you'll probably have your backsword recipe within an hour