Okay so if Glacius gets to cause strong freeze, Combuster gets to cause strong fire, then Acheron gets to cause... nothing?
Tell me why this wont be balanced compared to other brandish types, it needs some kind of elemental status.
Note: Fiends and Undead are immune to curse, and don't tell me "Because of Faust etc." since theres also Fang of Vog.
It's balanced because it inflicts a rare damage type. Currently, there are only four weapon lines that inflict shadow, (faust, graviton, shadowtech, acheron). There is no means of inflicting pure shadow via a sword, and the only other sword option that inflicts shadow (faust) is somewhat difficult to acquire and has a backdraft of occasionally cursing yourself.
Gremlin danger rooms account for a lot of difficulty in high tier play. Giving Acheron a curse effect would trivialize them in a way nothing else does.