While I do like the understated, subtle look of the nightblade line, the jump between Silent Nightblade and Acheron is disappointing. It gets an extra huge-toggly thing on the sheath. That's it. Compare that to how the fire & freeze brandish lines completely metamorphosize between each star level. It sounds kind of picky I'm sure, but I like it when my new item looks markedly different from the old one. That way I feel like I've accomplished something when I make it.

My suggestions on new appearance:
Make the sword slightly longer, and with a slightly stronger curve.
Cover the blade with purple stripes of varying widths. Bonus points if they are at random orientations and cross eachother occasionally.
Or: change the stripe to run down the side of the blade, either clear to the tip or stopping just before it.
Or: change the stripe to 'wrap around' the blade from hilt to tip, in a kind of spiraly thing.

Archmortal is spot on when he says that purple accent needs to be expanded on somehow. It's about the only focal feature of the sword, so some change in it gives the very satisfying 'brand new item' feeling that's currently missing from the Silent Nightblade -> Acheron transition.

Also, seconding Zeirnaton's comment. Let's use the sheath for something if it's going to be there, and charging an attack with it in the sheath is all kinds of classy.

I hate having a 5 star of this series and having people go "silent nightblade or acheron?" It is pretty annoying. That extra bulb added to the shealth is not enough especially with the shields that cover it almost entirely.