It can be acquired by typing
"player.additem 000771B9 X" into the console, where "X" is the quantity desired. After successfully typing the code, a message in the upper left hand corner reads, "Phantom Sword Added," but the weapon is not added to the inventory. The weapon has no stats and cannot be upgraded using a grindstone.
It can only be equipped by typing
"player.equipitem 000771B9" into the console. The weapon is then equipped and can be drawn, but the player is holding nothing at all. You can use it to kill animals and people, but there are no available stats and it vanishes as soon as you equip a different weapon or spell.
- When examining the game files through the Creation Kit, it is revealed that the Phantom Sword is used by the phantom-like Greybeard appearance which is summoned for a quest in High Hrothgar while the Dragonborn learns to use shouts. Likely, it is an invisible placeholder which prevents the summoned phantom from instead raising their fists, much like the ordinary Greybeards sometimes do when angered.