Night Sword Achievement in Vampire Survivors

The Night Sword is located directly north in Mt. Moonspell, and is clearly marked on your map. It is, quite frankly, a bit of a pain to get to. Navigating the north end of the map can be annoying with all the paths that wind around, rather than being an open space like all the other maps. To get to it, you need to head directly north from the starting area, up and through a cave with very strong enemies, and you'll eventually get to a little snow shrine with the Night Sword and a huge skeleton enemy guarding it. Simply pick it up for this achievement though, and then you'll be able to get it when leveling up with any character in any stage.

Alternatively, start the stage with the Mad Groove Arcana. This brings all items on the map to you immediately, including the Night Sword. Then, you can simply pick it up right next to you.