The story behind it, is that these Shadowdancers are actually the elite spies of some secret elven Order destined to preserve the survival of the
elves on Faerun (Faerunian elves like in most settings are being slowly pushed to extinction because of the rapidly increasing human expansion).
To cut a very very long story short, these elves managed to somehow steal information from Mask (Lord of Shadows - Lesser deity).
By combing High elven magic with Mask's portfolio, they managed to establish a significant link with the plane of shadow...
...so this is how those elves get their Shadowdancer (Su)s and (Sp)s...
Those magical abilities derive "purely" from their arcane knowledge and study...
With the sneak attack damage, I almost think that d4 HP is too little, for even a barely melee character. But d6 would give it regular rogue hp, without bringing it down a bit for the wizard/sorceror part.
I understand what you mean. I decided on d4 because i thought that even the Shadowdancer's class abilites are more related to a caster's than a rogue's. Don't get me wrong. They certenly are more useful to a rogue than a caster. It's just that they derive from magic as explained above. Makes some cense?
As most have suggested, I'd rather increase spells per day... but then again it's a change that can drastically alter the power of the PrC...
Well, with what I mentioned, it would be a +1. But I'm not sure what to get rid of to balance it out.
Ok..I'm NO more confussed!
On the scale you provided, there is no reason to strive for zero. Other existing PrCs raneg all the way up to +5 (Planar Shepherd). I think even if it is +1 with the changes, that's ok.
Yeah... perhaps you are right... I don't know. I get freakishly weird when it gets to things like this!!! I want it to be at 0, EXACTLY 0!
just kidding! (or...not...)