Milly Tanner, Arcane Trickster (Unchained Rogue/Greensting Slayer Magus)

The folks at Iconic Design (darn ya Alex.. you're truly awesome at this) brought up some excellent tricks with the Dirty Tactics Toolkit that make up a good way to save a few caster levels. Specifically one feat, Accomplished Sneak Attacker, which gives an additional 1d6 to sneak attack.
Why is this important you ask? Because it's the difference between 1 and 3 levels of rogue. This way you get the 1 level you need to make certain skills class skills (cause anyone who wants to disable locks is going to want to be able to do it +3 skill better)
For me there are two important things in this build:
- Accomplished Sneak Attacker: Dirty Tactics Toolkit
- Sense Vitals: A spell that lets adds +1d6 sneak attack/3 levels (5d6 @15th cap) to any manufactured weapon you attack with. NASTY. Add in the reddish glowing eyes you have while it's in effect for the creepy factor.
Class Selection: Unchained Rogue and Greensting Slayer Magus. I like the UC rogue a lot and if you decide to do a few levels dip instead of taking Accomplished Sneak Attacker, it has some nice benefits to 2 more levels of UC.
Greensting Slayer Magus is another source of sneak attack damage. Granted its only +1d6 with a swift action in the levels you get out of it but it's another 1d6.
The whole plan is to give the magus a bit more utility than outright 'touch attack with weapon attack', rinse, and repeat. With sneak attack damage and even more courtesy of her arcane pool and/or sense vitals, sneak attack makes up for not using as many spells to keep up the damage. Assuming a point of arcane pool being spent, Sense Vitals and normal sneak attack dice are in play; she can do 9d6 sneak attack in a single attack or 8d6 per attack during a full round of sneak attacking (assuming flanking). Fun note: the pool point only covers a single attack, so you can swap out Greensting for another archetype or even a vanilla magus. Though it does provide that extra bit of umph till you can get Spellbinding (Sense Vitals) at 7th. You can hold off till 9th level when you get 2 spells for the arcana but I'm impatient.

If you're lucky.. I'll just stab you

1st Level
Here we are at 1st level. Right now Milly is just a simple rogue, with a bit more intelligence than most and a hint of something arcane in her heritage (Blood of Dragons trait). She's got a wide variety of choices in weapon selection, with 14 Str/Dex, among any. For this particular build I'll be working towards a bastard sword but with a little 'flexibility' in skill selection could have enough dance to pick out things like Dervish Dance (Which would allow him to use a scimitar and apply dex to damage, if you want to tweak out strength for more dex).

Milly Tanner
Female human (Varisian) rogue (unchained) 1 (Pathfinder Unchained 20)
N Medium humanoid (human)
Init +2; Senses low-light vision; Perception +4
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Defense
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AC 12, touch 12, flat-footed 10 (+2 Dex)
hp 10 (1d8+2)
Fort +2, Ref +4, Will +0
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Offense
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Speed 30 ft.
Special Attacks sneak attack (unchained) +1d6
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Statistics
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Str 14, Dex 14, Con 14, Int 16, Wis 10, Cha 10
Base Atk +0; CMB +2; CMD 14
Feats Combat Expertise, Fast Learner[ARG], Weapon Finesse
Traits blood of dragons, killer
Skills Acrobatics +6, Diplomacy +4, Disable Device +7, Disguise +4, Escape Artist +6, Knowledge (arcana) +4, Knowledge (local) +7, Linguistics +7, Perception +4, Sleight of Hand +6, Stealth +6, Use Magic Device +4
Languages Aquan, Common, Draconic, Kelish, Osiriani, Varisian
SQ trapfinding +1
Other Gear 150 gp
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Special Abilities
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Combat Expertise +/-1 Bonus to AC in exchange for an equal penalty to attack.
Low-Light Vision See twice as far as a human in dim light, distinguishing color and detail.
Sneak Attack (Unchained) +1d6 Attacks deal extra dam if flank foe or if foe is flat-footed.
Trapfinding +1 Gain a bonus to find or disable traps, including magical ones.

4th Level
Here we are at 4th level, with 3 levels of greensting magus for another die of sneak attack (if you use an arcana point). She can do sneak attack, spell combat and with close range to let her use spells like ray of enfeeblement to debuff her foes. Toss in vanish for 'instant' sneak attacks in the next round.

Milly Tanner
Female human (Varisian) magus (greensting slayer) 3/rogue (unchained) 1 (Pathfinder Player Companion: Bastards of Golarion, Pathfinder RPG Ultimate Magic 9, Pathfinder Unchained 20)
N Medium humanoid (human)
Init +2; Senses low-light vision; Perception +6
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Defense
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AC 12, touch 12, flat-footed 10 (+2 Dex)
hp 34 (4d8+11)
Fort +5, Ref +5, Will +3
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Offense
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Speed 30 ft.
Special Attacks magus arcana (close range[UM]), sneak attack (unchained) +1d6, spell combat, spellstrike
Magus (Greensting Slayer) Spells Prepared (CL 3rd; concentration +6)
   1st—chill touch (DC 14), ray of enfeeblement (DC 14), shocking grasp, vanish[APG] (DC 14)
   0 (at will)—acid splash, detect magic, mage hand, prestidigitation
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Statistics
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Str 14, Dex 14, Con 14, Int 17, Wis 10, Cha 10
Base Atk +2; CMB +4; CMD 16
Feats Combat Expertise, Exotic Weapon Proficiency (bastard sword), Fast Learner[ARG], Weapon Finesse
Traits blood of dragons, killer
Skills Acrobatics +9, Diplomacy +4, Disable Device +10, Disguise +4, Escape Artist +9, Knowledge (arcana) +10, Knowledge (local) +7, Linguistics +9, Perception +6, Spellcraft +8, Stealth +9, Use Magic Device +5
Languages Aquan, Auran, Common, Draconic, Ignan, Kelish, Osiriani, Varisian
SQ arcane pool, trapfinding +1
Other Gear 150 gp
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Special Abilities
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Arcane Pool (+1d6 sneak attack, 4/day) (Su) Expend Arcane Pool to add sneak attack dmg to next melee atk.
Close Range (Ex) Using spellstrike, deliver ranged touch spells as melee touch spells.
Combat Expertise +/-1 Bonus to AC in exchange for an equal penalty to attack.
Low-Light Vision See twice as far as a human in dim light, distinguishing color and detail.
Sneak Attack (Unchained) +1d6 Attacks deal extra dam if flank foe or if foe is flat-footed.
Spell Combat (Ex) Use a weapon with one hand at -2 and cast a spell with the other.
Spellstrike (Su) Deliver touch spells as part of a melee attack.
Trapfinding +1 Gain a bonus to find or disable traps, including magical ones.
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7th Level
Now we have Milly as a full on Arcane Trickster, thanks to putting in Accomplished Sneak Attacker @ 5th level for that 2nd die of sneak attack that we need for Arcane Trickster. Thanks to that we were able to hold off on 2 levels of rogue and get more spell levels earlier. So at 7th level, she can get Spellblending (1 spell: Sense Vitals) for even more sneak attack (3d6 from classes/feats +1d6 (greensting sneak attack) +2d6 (Sense Vitals)). It does require a MANUFACTURED weapon.

Milly Tanner
Female human (Varisian) arcane trickster 2/magus (greensting slayer) 4/rogue (unchained) 1 (Pathfinder Player Companion: Bastards of Golarion, Pathfinder RPG Ultimate Magic 9, Pathfinder Unchained 20)
N Medium humanoid (human)
Init +2; Senses low-light vision; Perception +10
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Defense
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AC 12, touch 12, flat-footed 10 (+2 Dex)
hp 54 (7 HD; 2d6+5d8+18)
Fort +7, Ref +6, Will +5
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Offense
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Speed 30 ft.
Melee bastard sword +6 (1d10+2/19-20 plus 1d6 sneak attack plus 1d6 sneak attack)
Special Attacks magus arcana (close range[UM], spell blending [1 spell][UM]), sneak attack (unchained) +1d6, sneak attack +2d6, spell combat, spell recall, spellstrike
Magus (Greensting Slayer) Spells Prepared (CL 6th; concentration +9)
   2nd—brow gasher[UC], darkness, mirror image, scorching ray
   1st—chill touch (DC 14), ray of enfeeblement (2, DC 14), shocking grasp, vanish[APG] (DC 14)
   0 (at will)—acid splash, detect magic, mage hand, prestidigitation, read magic
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Statistics
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Str 14, Dex 14, Con 14, Int 17, Wis 10, Cha 10
Base Atk +4; CMB +6; CMD 18
Feats Accomplished Sneak Attacker, Combat Expertise, Exotic Weapon Proficiency (bastard sword), Extra Arcana[UM], Fast Learner[ARG], Weapon Finesse
Traits blood of dragons, killer
Skills Acrobatics +12, Bluff +4, Climb +6, Diplomacy +4, Disable Device +13, Disguise +4, Escape Artist +9, Knowledge (arcana) +10, Knowledge (local) +8, Linguistics +13, Perception +10, Spellcraft +8, Stealth +12, Use Magic Device +8
Languages Aquan, Auran, Azlanti, Common, Draconic, Ignan, Kelish, Osiriani, Sylvan, Terran, Thassilonian, Varisian
SQ arcane pool, ranged legerdemain, trapfinding +1
Other Gear bastard sword, 115 gp
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Special Abilities
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Arcane Pool (+1d6 sneak attack, 5/day) (Su) Expend Arcane Pool to add sneak attack dmg to next melee atk.
Close Range (Ex) Using spellstrike, deliver ranged touch spells as melee touch spells.
Combat Expertise +/-2 Bonus to AC in exchange for an equal penalty to attack.
Low-Light Vision See twice as far as a human in dim light, distinguishing color and detail.
Ranged Legerdemain (Su) Can use Disable Device or Sleight of Hand from 30 ft away, but +5 DC.
Sneak Attack (Unchained) +1d6 Attacks deal extra dam if flank foe or if foe is flat-footed.
Sneak Attack +2d6 Attacks deal extra dam if flank foe or if foe is flat-footed.
Spell Combat (Ex) Use a weapon with one hand at -2 and cast a spell with the other.
Spell Recall (Su) Spend your Arcane Pool to recall spells you have already cast.
Spellstrike (Su) Deliver touch spells as part of a melee attack.
Trapfinding +1 Gain a bonus to find or disable traps, including magical ones.
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12th Level
Now at seeker level, Milly is a sneak attacking monster with skills in lock picking, magic device use, and so on. She can, when prepared, do HUGE sneak attack damage (5d6 (class)+ 1d6 (optional, Greensting Arcana) + 3d6 (Sense vitals)). Adding in fun things like Arden's spellsword (for storing a metamagic rod or wand/rod/staff of her choice) and spells like Greater Invisibility, Displacement, Blur, Mirror Image and such.
There are a variety of tactics you can use. Things like using invisibility/greater invisibility to summoning backup and such for the rest of the part. Add in two extra impromptu sneak attacks (courtesy of the feat x2) and you can do a bunch of sneak attack damage on demand. Tricky Spells means you can cast still/silent spells up to 4 times a day.

Milly Tanner
Female human (Varisian) arcane trickster 7/magus (greensting slayer) 4/rogue (unchained) 1 (Pathfinder Player Companion: Bastards of Golarion, Pathfinder RPG Ultimate Magic 9, Pathfinder Unchained 20)
N Medium humanoid (human)
Init +3; Senses low-light vision; Perception +15
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Defense
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AC 22, touch 15, flat-footed 19 (+7 armor, +2 deflection, +3 Dex)
hp 96 (12 HD; 7d6+5d8+40)
Fort +12, Ref +13, Will +11
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Offense
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Speed 30 ft.
Melee +2 dispelling keen adamantine bastard sword +11/+6 (1d10+5/17-20 plus 1d6 sneak attack plus 3d6 sneak attack)
Special Attacks magus arcana (close range[UM], spell blending [1 spell][UM]), sneak attack (unchained) +1d6, sneak attack +4d6, spell combat, spell recall, spellstrike
Magus (Greensting Slayer) Spells Prepared (CL 11th; concentration +17)
   4th—greater invisibility (3)
   3rd—aroden’s spellsword, displacement, fly, haste, keen edge
   2nd—mirror image (2), scorching ray, sense vitals (3)
   1st—ray of enfeeblement (3, DC 17), shocking grasp (2), vanish[APG] (2, DC 17)
   0 (at will)—acid splash, detect magic, mage hand, prestidigitation, read magic
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Statistics
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Str 16, Dex 16, Con 16, Int 23, Wis 10, Cha 10
Base Atk +6; CMB +9; CMD 24
Feats Accomplished Sneak Attacker, Combat Expertise, Exotic Weapon Proficiency (bastard sword), Extra Arcana[UM], Extra Impromptu Sneak Attack, Extra Impromptu Sneak Attack, Fast Learner[ARG], Weapon Finesse
Traits blood of dragons, killer
Skills Acrobatics +18, Bluff +4, Climb +7, Diplomacy +5, Disable Device +19, Disguise +4, Escape Artist +10, Knowledge (arcana) +21, Knowledge (local) +17, Knowledge (nature) +21, Knowledge (planes) +21, Linguistics +21, Perception +15, Spellcraft +16, Stealth +23, Use Magic Device +15
Languages Abyssal, Aquan, Auran, Azlanti, Common, Draconic, Elven, Hallit, Ignan, Kelish, Osiriani, Shadowtongue, Shoanti, Skald, Sylvan, Terran, Thassilonian, Tien, Undercommon, Varisian
SQ arcane pool, impromptu sneak attack, ranged legerdemain, trapfinding +1, tricky spells
Combat Gear extend metamagic rod (lesser); Other Gear +3 shadow mithral chain shirt, +2 dispelling keen adamantine bastard sword, belt of physical perfection +2, cloak of resistance +3, headband of aerial agility (int +4)[UE], ring of protection +2, 150 gp
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Special Abilities
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Arcane Pool (+1d6 sneak attack, 8/day) (Su) Expend Arcane Pool to add sneak attack dmg to next melee atk.
Close Range (Ex) Using spellstrike, deliver ranged touch spells as melee touch spells.
Combat Expertise +/-2 Bonus to AC in exchange for an equal penalty to attack.
Impromptu Sneak Attack (4/day) (Ex) You can declare any melee or ranged attack to be a sneak attack.
Low-Light Vision See twice as far as a human in dim light, distinguishing color and detail.
Ranged Legerdemain (Su) Can use Disable Device or Sleight of Hand from 30 ft away, but +5 DC.
Sneak Attack (Unchained) +1d6 Attacks deal extra dam if flank foe or if foe is flat-footed.
Sneak Attack +4d6 Attacks deal extra dam if flank foe or if foe is flat-footed.
Spell Combat (Ex) Use a weapon with one hand at -2 and cast a spell with the other.
Spell Recall (Su) Spend your Arcane Pool to recall spells you have already cast.
Spellstrike (Su) Deliver touch spells as part of a melee attack.
Trapfinding +1 Gain a bonus to find or disable traps, including magical ones.
Tricky Spells (4/day) (Su) You can cast a spell without verbal or somatic components.

Notes: There was a variety options here. Swap out greensting for a normal magus. Swap out EWP: Bastard sword for a scimitar and Dervish Dance. Wait till 9th level to get spell blending to get TWO spells rather one.