to spice it up a bit Make the entire amount of Damage shadow damage
penalty Vs undead high so you really can't hurt undead with it

and the charge attack should not unleash energy balls but maybe make it look like it does something else
I'm thinking spectral Shadow Hands clawing at the enemy as the after image attack and bigger ones for the charge attack
OR since you like the phantoms have a spectral overlay on top of the character of a phantom slashing with you
___or in the idea have a phantom slashing the enemy with you but, it circles the hit zone for the enemy starts off where you are and hits then jumps right and circles right again and then circles left and salutes you as it disappears and makes an explosion (very small one)

put a phantom skull sorta on the blade where the design goes
give it the dread venom's poison aura and pallet swap it to shadow purple
make there be a minor chance of major curse on the 5 star

we need to come up with better names for the 4 and 5 star
for the 5 star I was thinking something like Phantasmal Ripper
and for the 4 star name it Darkling's Shredder

I like the idea of the phantom's laugh/giggle with every attack maybe have the "huurrr" sound (the sound when they respawn) at the start of the charge attack
Edit: bump.