I really enjoy the historic selection of armors...why do we have such an odd sword?

[MagpieOAO]

And only one kind? 

The flavor text indicates a flax is better against unnatural creatures. My understanding of the historic weapon is it was vicious and particularly detested for its ability to hit over shields. 

[LadyWYT]

I would assume it's mainly a design choice, in order to have something a bit more unique than just a plain sword. From a practical standpoint, the design seems to offer decent slashing ability, as well as the ability to focus the energy of your swing into the blade's tip if you so choose, allowing it to penetrate deeply. The saw-toothed portion can potentially make a bad piercing injury even worse if you twist the blade when drawing it out of the victim, tearing open the wound further. The blade's design is also similar to common farming implements like scythes and sickles, perhaps making them easier to produce en masse. A lot of Falxian forces seem to be pulled from the common people, and farm tools would be readily available, as well as unneeded in underground bunkers.

[Never Jhonsen]

There's a bit of lore as to why the Falx Blade looks like that

Spoiler

To quote from the Wiki;

Quote

"Falx" can also refer to the inventor Jonas Falx. It is not known what role Falx played in the introduction of this weapon.

Item description :
The creatures hide metal in their flesh and their blood flows like sludge. A simple sword will not do here.

This description suggests that the weapon was designed to fight drifters. The mention of metallic flesh parallels historical reports of Romans needing to modify their armor against falx-wielding Dacian forces.

Jonas Falx is a very important character in the game's story. Several Lore Books mention his name, but most important of all is the five-part narrative entitled Reflections.

It used to be a normal, straightish sword, 1.17 was the update that changed it to the falx.

[Maelstrom]

There also remains the straight bladed Blackguard sword, unique item for blackguards.

[Facethief]

Also, Falxes are a real type of sword- they actually look like that, aside from the serrated bit.

Considering this, the man (Jonas) is actually named after the sword. How unfortunate.

[Maelstrom]

Jonas' father has the surname Falx who was a mercenary turned noble whose son (Jonas) creates the falx to better fight drifters and the sword is subesquently named after him.  I think it is a fitting literary device.

[Facethief]

1 hour ago, Maelstrom said:

Jonas' father has the surname Falx who was a mercenary turned noble …

The father or one of his ancestors likely utilized the falx extensively in their mercenary work, which would explain this.

1 hour ago, Maelstrom said:

… (Jonas) creates the falx to better fight drifters and the sword is subesquently named after him.

But the sword is a pre-existing variety? Jonas likely realizes the anti-armor use of his father’s favorite weapon and brings it into use against the rusted.