I just remembered
this prestige class, although I doubt it is what you're looking for. It might give you some ideas for class features, though.
If I were doing this, I'd make this a more "generic" class, not a specific character. I'd probably take some of the things from Nullblade, because the whole void/nullifying thing seems like it fits somewhat with the powers and fluff of the River Styx, but not completely. I'd add in some abilities about navigating and swimming, immunity to the effects of the River Styx at low levels, and at higher levels let this increase to total immunity to Mind Affecting things. Maybe add in some of the stuff from the Boatman of the Styx about bottling your tears and using them as a weapon that drains memories and/or stuns people or something.
I also had an idea that some of the powers of the
Mindstealer Drone might be fitting, as the Styx is supposed to absorb the memories it trains, and supposedly they're still there to be accessed by someone who knows how (and who else might but Charon?). Negative levels and energy also seem fitting, as Charon is typically portrayed as being skeletal.
So, maybe something like this overall:
Take the Nullblade class, change the Void Strike to be negative energy. Give Ancestral Relic as a bonus feat, and specify that it is a Nullcraft weapon (see Nullblade items). Modify the Invocation list to include things like Modify Memory, Mind Wipe, etc, that have to do with erasing someone's memory.
At X level (maybe 7th), have each of the boatman's (or whatever you name it) attacks with his oar deal a negative level, and give the boatman the ability to absorb the memories of those he deals negative levels to, as the Mindstealer Drone ability. Let this increase in potency, at first gaining only skill ranks, then feats, and then spells or spell-like abilities, much like the Spellthief.
In fact... maybe
Spellthief is a better chassis for this whole thing in the first place! Hell yes, it is! Use the one that gets bardic spellcasting, and it's almost perfect "out of the box", in fact, if you just add on the Ancestral Relic thing, and maybe something about negative energy damage/energy drain, and stealing memories (skills, memories, and feats, the class already steals spells and spell-like abilities).
Anyway, just some ideas that popped into my head. I kind of like the idea of coming up with a Boatman of the Styx base class and thought I'd throw some of this out there.