It looks like what this fellow did was enable cheatmode and open the inventory and pick the top statted items. you can't obtain many of these things legitimately, i think you have to spouse one of the vassals to get that sword, and the "pureblood" horse is easy to get from your siblings who have them in their civilian sets.
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
I don't know how many types of engraved backswords there are, but they are common side arm on lords, sometimes in the civilian slot too. If you make a female char you can mary and hunting accident husbands and keep getting more gear until you find the one you're looking for.
Play as a female character. Marry a Battanian man.
Battanian nobles often carry an engraved backsword in their town outfits.
Off the top of my mind, I married Sein. Weird looking guy, but had some good kit. I sold it for 35k. Sein cried so I gave him a rusty falchion. Annoyingly, after I sold it it sat in the shop for 99k. Inflation. Sigh.