There seems to be a lack of shadow weapons in comparison to elemental and piercing weapons, there are only like, 2 sword lines that deal shadow, and 3 (I think), gun lines. Bombs have a decent amount of shadow weapons, but the amount of shadow swords and guns is a bit too little.

Due to the gunner update being soon, I am going to focus on suggesting new swords, mainly because I do not know what damage types the new guns will inflict.

Since we already have a slow shadow sword (faust line) and a medium shadow sword (nightblade line), I think we need a fast shadow sword.
I've decided to make it a skele themed line.
The alchemy path is:
Regular line: Skele Blade ---> Scary Skele Blade ---> Sinister Skele Blade ---> Dread Skelly Blade
Branch line: (branches at 4 star) ---> Slag Walker Sword

Skele Blade:

Base attack:
A 4-hit combo with a fast speed that deals
a small amount of shadow damage, the first 3
swings do not move you, but forth causes you to
leap forward a tile dealing shadow damage to anything
you hit (sorta like a smaller WRH dash)
Charge attack:
Your knight jumps backwards and then dashes forwards dealing
shadow damage. (Both the backwards and forward dash do damage)

The scary and sinister skele blades do the same thing,
but deals more damage per star level.

Dread Skelly Blade:

Base attack:
A 5-hit combo that deals a large amount of shadow
damage, the 4th hit does the same as the other skele blades,
the fifth is a wide swing after the knight finishes dashing.
Unlike the other skele Blades, the Dread skelly blade has a slight
chance to deal curse on the fifth hit.

Charge attack:
Simlar to the other skele blades, but dashes deal more
damage and have a moderate chance to curse the victim.

Alternatively, you can craft the Slag walker sword, but note that the recipe costs 25 almirian seals to obtain from the trader.

Slag Walker Sword:

Base attack:
A 4-hit combo (same as previous skele blades) that has a moderate chance to deal fire on the fourth hit, not only
to enemies. Note that this sword also deals more damage, but is slightly slower in executing combos.

Charge attack:
The user dashes forward, dealing shadow damage and sometimes inflicting fire on enemies, then dashes back
and attacks with a 180 degree slash that will leave the knight facing the opposite direction as they were facing pre-charge attack.

The recipes for everything, other than the slag walker sword, can be purchased from Basil at the usual rate.

Hope you liked my idea, and sorry for the massive wall of text.
Feel free to add any constructive criticism.