There are quite a bit of rare weapons in Fallout 4 like the Shem Drowne sword. Like other rare or hidden weapons, you can only get this sword during The Gilded Grasshopper quest, which is available to get on the PC, Xbox One and PS4 version of the game.
For those looking for the Shem Drowne sword that deals radiation damage, you will have to take up a quest that will lead you to his grave, which contains a few extra goodies in his casket. You will want to go to Diamond city and then go to Nick Valentine’s office and get the quest “The Gilded Grasshopper” off of a cabinet shelf. After listening to the holotape on Valentine’s shelf you will be able to go to the next location, which is Faneuil Hall.
To get to the this abandon hall you will need to go north-east of Diamond city and just north-west of the Custom House Tower. Once there you will encounter a lot of tough super mutants in Faneuil Hall, which also has a chance of spawning legendary super mutants.
After clearing the building out, you will need to head to the roof and navigate across some scaffolding to obtain the actual Gilded Grasshopper item. After picking the quest item up, you will be given a note that provides a marker on your Pip-boy showing Shem Drowne’s grave location, which is just north of Faneuil Hall.
Upon reaching the graveyard you will have the option to dig up Shem Drown’s grave, you will need to do so in order to get the sword from his casket. In addition to this, he will have different types of gold and silver bars that are worth taking. Furthermore, if you need to see this in action you can check out TwoDynamic‘s guide on how to obtain Shem Drowne’s sword.
Lastly, for those that are curious about the sword’s stats and want to know if it’s worth skipping or getting, the Shem Drowne has a base damage of 33, radiation damage of 9, a weight of three, and a value of 250. Those that like to upgrade weapons will find that you can add electrical damage on the sword as well as a chance to stun enemies during combat when modding it. So Shem Drowne sword is good for low to mid-level enemies, and will be most useful during the beginning parts of Fallout 4.