I don't recommend it lol. The sword is kind of a meme. I mean... its not a bad weapon... but most of the time there are better weapons lol. You can get it right above the mountains that are above Arise village (mid southish area) passed the cave. You need to complete a challenge where you face a bunch of undead and then finally the undead boss. Be wary, this boss hurts and is hard. Traps are hard to use as the boss has 3x3 AOE attacks that clears traps, has a charge ability to prevent bow cheesing (still can cheese with bow if you can time it right) and hits like a truck. Bring plenty of potions for emergency heals and sta recovery. The boss is tanky and is rather fast so careful.
Once you defeat the boss, the sword drops at the center of the map and you can claim it. It weighs 40 so make sure you have enough inventory capacity or else is gonna be rough walk back.
Best way to defeat the boss is to bow cheese via snipe. Once the boss has the angry sign on it, dodge and run around until he goes back to his usual spin attacks. Rinse and repeat.
Traps can also work well if you are undetected and place lots of traps before you get into his attack range.
Do not engage in CQC with him as his AOE damage will shred through you and give you untold amounts of injuries. Also since he is an undead, stuns don't work on him.
I've never tried to bring a companion to this challenge so I cannot tell you which one would be helpful or if they will be useful at all.