Magus / ninja / shadowdancer sounds like the sort of multiclass which doesn't advance enough class features and which you will regret in a few levels. Do you mean magus // ninja / shadowdancer? The double slash is used to show different sides of the gestalt.
If so, it sounds a little MAD but probably doable. Possible swift action conflict between many magus arcana and tricks like vanishing trick or flurry of stars though. Also 'martial' for a gestalt character usually wants better than 3/4 BAB. And, IMO, shadowdancer is no more shadowy than more ninja levels.
How about Arcane Bloodrager // Ninja? Full BAB, casting and ki both based off charisma, less action conflict, and from 4th level you can be automatically blurred when in a bloodrage for that shadowy feel. The metamagic rager archetype might be handy if you want to cast spells silently (besides any triggered just by entering a bloodrage.)
|What's in the box?|
If Advanced Races Guide is allowed: Fetchling or Wayang are shadow themed races. With Fetchling being my preferred race (I am currently playing a Fire/Shadow themed Fetchling in an evil campaign).
If you are not allowed to use that book or those races I think Half-elf might have modicum of flavor (i.e. despised race that lurks in the shadow) and you could spec to be a half Drow to amp up the evil and darkness a notch as well.
I think your build may be difficult because you are combining multiple 3/4s BAB classes... I feel like you will be able to do AWESOME stuff... if you EVER hit (this build puts you at a -3 for BAB progression). If you have a way around that (like a LOT of gold for super weapons OR templates you can get to boost Attack- then COOL!)
If you are looking to play a gish-type player the Inquisitor class has some utility in being able to use magic and hit decently- they get judgments and bane for weapons too which can balance (partially) with a magus spell abilities. That could pair well with a progression into Shadow Dancer.
Alternatively I don't think you would have to pair magus with ninja if you were trying for mystical shadow fighter. ninjas already have that and picking a couple of cool ninja tricks (there is one that makes duplicates, one that makes you invisible) that have mystic/shadowy essence to them.
You may even look at the new Slayer hybrid class to see if that can be appropriately themed (I mention since it has a full BAB).
The shadowdancer was there to easily get and use the dimensional agility feats later, but I see your point.
And yes it's Magus//Ninja with shadowdancer. Was going to level it like this:
9 9 etc.
Since it's gestalt, kinda wierd sticking a PrC in there...
|What's in the box?|
OH... Sorry, I didn't realize Gestalt.
I am not sure that the progression is allowed. Typically Gestalt rules prohibit progressing in a Prestige class on both sides. But if you have a house rule then I think you are fine. Hitting may still be a problem (Gestalt foes tend to be SO FRIGGIN AMPED!!!)
My fetchling was having some issues in a recent solo encounter session... To be fair I am level 13 and it was a CR 18 (that I DID solo- so yay me!) but it was a friggin nightmare...
I also roll for crap though... so there is that.
I don't think you can advance a class twice as fast by putting it on both sides of a gestalt. That's just not the way gestalt works.
Dimensional Agility. OK, changing my plan.
With one of your ninja tricks take rogue trick: terrain mastery. The plan is now Arcane Bloodrager 6 / Horizon Walker 3 / more Bloodrager // Ninja. You avoid delaying master tricks & sneak attack, you keep full BAB, though admittedly you delay bloodrager class features.