I am trying to make an assassin type character that gets into melee like a rogue but has illusion based magic as back-up. I'd plan on using such spells like invisibility, the image spells, and hopefully some of the shadow spells. Currently some very rough ideas I threw together were:
Bard (Sandman) / Crimson Assassin(RMA) / Shadowdancer[I like the whole shade thing but I am unsure if it would work]
Rogue / Wizard (Shadowcaster) / Arcane Trickster
Ninja / Sorcerer (Shadow 3rd party) / Arcane Trickster
Link to the 3rd party bloodline: Shadow Bloodline
The Arcane Trickster PrC seemed like something to what I was looking for, but unfortunately with their low BAB it seems like you would end up spamming rays most of the time (I am willing to fall back on this if the whole melee thing ends up being a dead end) instead of being in melee getting off sneak attacks. I've been playing mostly blasters recently so anything to get away from that kind of feel would be nice.
Another option would be to go straight Sandman Bard, however with such infrequent sneak attack bonuses I feel like my damage would be rather pathetic in melee. I like the feel of sneak attacks although I could be convinced to just take power attack etc and rely on dervish dance or something to deal melee damage.
The character doesn't need to be optimized, as long as it can function well enough in combat. Currently we plan to go from level 1 to as high into the teens as we can go, hopefully until 20. My DM is rather nice when it comes to not killing our characters, he won't focus me just because I am there flanking kind of thing so I feel like I have a little more wiggle room to work with than usual.
I know this is all really rough and lacks a lot of fine details but any advice or ideas you have will be welcome.
Edit: My DM wanted me to stick to official PF stuff for the most part, 3.5 stuff is generally out of the question unless I clear it with him first (same with 3rd party)
Vivisectionist alchemist. If a formulae (read: spell) isn't on the list, then ask your DM for a substitute. As long as it isn't game breaking, I'm sure they won't have a problem.
I'd say the same for magus, but you won't get sneak attack.
I think I would lean towards straight Ninja. I like to avoid multiclassing as much as possible in Pathfinder, and it seems like you want melee to be your primary skill. They cover the invisibility aspect of illusion about as well as any caster, and they can get a couple other abilities with an illusion-like feel to them.
Don't forget that you can abuse Use Magic Device to equip yourself with a variety of spells from wands and staves. The Image spells often don't require saves, so a wand could work fine, or you could spring for a staff at higher levels. Personally, I'm not a big fan of the Shadow spells as anything other than utility spells, so save DCs might not matter there either. Ninjas like to have good charisma, so this works well with that class.
Bard and Alchemist could also be what you want. Bards are excellent at illusion magic, but sacrifice a lot in the assassin aspect I think you're looking for. Definitely look into the Dervish Dancer or Dervish of Dawn archetypes, they both sacrifice some of your ability to buff for greater melee skill. Vivisectionist Alchemists make very good rogue-replacements, but their spell list is pretty sparse on illusions.
If you aren't as concerned with SA, I might try going ninja 10/ shadowdancer 10 (assuming you can get that high. You will still be able to deal 5d6 SA, can gain ninja & rogue tricks, and get a fast track on killing things by having a companion that deals STR dmg.
You can even contribute to that with tricks like pressure points (ninja) & crippling strike (rogue, advanced). Not to mention, the ninja's advanced trick assassinate is pretty nice, if circumventable.
& it gives you a nice bit of variety of illusion, evocation, and conjuration (if at reduced effect) with the shadow abilities, which can be boosted using the Eldritch Heritage line of feats and the Shadow blood long you liked to. An extra 10% real is pretty useful with those spell effects, though it'd be nice if they had a feat that would grant the capstone ability. Too bad it'd probably have to be an epic feat. :/
Thank you for all the suggestions so far - I hadn't even thought about going straight ninja or ninja / Shadowdancer, in fact I pretty much forgot about their ninja tricks, I'll take a harder look at the class, it could be exactly what I want.