Everybody knows that the Acheron line has hilarious damage output. How about changing the combo animation and hitboxes so that it consists of three vertical (up and down) swings instead of horizontal (side to side)? The hitboxes would basically be just lines sticking straight out from the knight, meaning they would only hit 1 enemy at a time, or rarely 2 if the conditions are right. This would act as both an indirect nerf to Acheron, and help separate the niches of Acheron and Gran Faust.
I feel that this would also fit Acheron aesthetically, since it looks completely different from any of the status Brandishes, has a very high damage output compared to the others, and has different "ideal" UVs (ASI instead of CTR). As it is now it's just "different" from the other Brandishes for no particular reason, it's just a special snowflake of a sword. This change would help justify those differences somewhat.
While interesting, I would prefer the acheron simply having the charge attack deal additional damage (current values) while the standard attack deals the same damage as the obsidian edge. The only thing that really changes for every brandish variant is its charge attack, so it feels like an oversight for one line to act entirely different at 5*.