Sword Wraiths?

[Shaonir]

Looking at these guys in Mordenkainen's Tome of Foes, and it seems like they don't have incorporeal movement (despite the ghostly pictures).

Can I assume these are more like Wights, with bones and spoopy lights underneath their robes? I'm picturing them kinda like underpowered LOTR Ring Wraiths, with physical forms that can be ruptured in order to spill their lifeforce.

If anyone has any additional lore, or has seen the answer somewhere, you shall be handsomely rewarded. So very handsomely...

[Sparky McDibben]

I kind of pictured them as the Dead Men of Dunharrow, from the same IP. I always run them as these berserk spirits who slumber until awoken, but once they wake up, they're overcome by battle-lust. Therefore, they're unlikely to try any incorporeal shenanigans.

Alternatively, you can run them as good-aligned warrior spirits bound to guard a place or person, sort of like a guardian draugr, vis-a-vis Skyrim.