No fluff. All smooth. Like a polished rock. Sleep well, children.

Quote Originally Posted by Fluffless
N Beguiler 1/Shadowcaster 3/Noctumancer 2
Spoiler: Racials

Whisper gnome (RoS 96)
+2 DEX/CON, -2 STR/CHA, Small (+1 AC/Attack, +4 Hide) Speed 30
Lowlight and Darkvision 60ft, Weapon Familiarity, +1 attacks vs kobold/goblinoids, +4 dodge to AC vs giants, +4 Hide/Move Silent, +2 Listen/Spot. SLA’s: 1/day Silence, Ghost sound, Mage Hand, Message
4d6+2d4+12 HP
Point Buy:
STR 11 DEX 14 CON 12 INT 17 WIS 10 CHA 12
STR 9 DEX 16 CON 14 INT 17 WIS 10 CHA 10
LVL 4 into INT

Spoiler: Build:

Level Class Base Attack Bonus Fort Save Ref Save Will Save Skills Feats Class Features
1st Beguiler +0 +0 +0 +2 Bluff 4, Conc 4, Hide 4, Know Arcane 4, Move 4, Sleight 4, Spellcraft 4, Tumble 4, UMD 4 Precocious Apprentice – Distracting Ray Armored Mage (Light), Trapfinding, Spells
2 Shadowcaster +0 +2 +0 +4 Conc 5, Hide 5, Know Arc 5, Move 5, Spellcraft 5 - Fundamentals, Apprentice Mysteries
3rd Shadowcaster 2 +1 +3 +0 +5 Conc 6, Hide 6, Know Arc 6, Move 6, Spellcraft 6 Shadow Cast, (B)Still Mystery -
4th Shadowcaster 3 +1 +3 +1 +5 Conc 7, Hide 7, Move 7, Spellcraft 7, cc-Bluff 5 - Umbral Sight (+30ft)
5th Noctumancer +1 +5 +1 +7 Conc 8, Spellcraft 8, cc-Tumble 5, cc-UMD 5 - Capture Magic’s Shadow, +1 SC/Mysteries
6th Noctumancer 2 +2 +6 +1 +8 Conc 9, Spellcraft 9, cc-Tumble 6, cc-UMD 6 Shadow Familiar, (B)Alertness Innate Counterspell 1/day, +1 SC/Mysteries

Spoiler: Feats/Spells/Mysteries

1-Precocious Apprentice – 2nd level casting
3- Shadow Cast – pick an unthreatened adjacent square (behind you) no AoO for casting/sla’s
3- Still Mystery – no somatic component for mysteries, they can’t be identified now
6- Shadow Familiar – gain a familiar w/ the dark template at full master level
6- Alertness - +2 listen/spot
1- Practiced Spellcaster (Beguiler) +4 CL up to HD, so CL 6
2- Improved Familiar
3- Martial Study (Action Before Thought) – use Concentration for REF saves
4- Shape Soulmeld (Lamia Belt) - +4 to Bluff/Hide
5- Favored Mystery (Sight Eclipsed) – becomes Su, 3/day
6- Extra Silence – 3 uses of racial Silence
7- Favored Mystery (Thoughts of Shadow) – becomes Su, 3/day
8- Battle Caster – can now cast in MED armor w/o ASF
9- Darkstalker – creatures w/ special senses must make listen/spot to see you
10- Able Sniper - +2 on ranged attack vs FF 30+ft away, gain +4 on hide checks after attacking
[b]Spells/Mysteries per day
3rd lvl Arcane at CL 6:]/b]
1: 0-5, 1-4, 2-1
5: 0-6, 1-5, 2-1
6: 0-6, 1-6, 2-1

Depending on how you read the bonus spells per day entry in the PHB p8, the ability to cast 2nd lvl spells is the determining factor for bonus spells/day. Since you can cast a 2nd lvl spell at 1st lvl through the Precocious Apprentice Feat, you should qualify for a bonus slot of 2nd level due to a high INT. This is not reflected in the table, but should you agree with that reasoning, add 1 to each 2nd lvl spell slot at all levels.
Beguiler gives you access to Abjuration, Conjuration, Divination, Enchantment, Evocation, Illusion, and Transmutation at lvl 1. Spell Selection for Precocious Apprentice: Distracting Ray (SpC 69): no save, no SR close range, ranged touch that forces a DC17+spell lvl concentration check or spell being cast is lost.
3rd lvl Fundamentals/Mysteries:
Fundamentals usable 3/day, Mysteries 1/day
2: Arrow of Dusk, Sight Obscured 2x, Steel Shadows
3: Mesmerizing Shade
4: Thoughts of Shadow
5: Arrow of Dusk +1, Afraid of the Dark
6: Sight Eclipsed
Arrow of Dusk (taken twice) – Med range ray, no save, no SR, 2d4 nonlethal, crits x3
Sight Obscured (taken twice) – 1rd/lvl +5 hide/sleight or other check to conceal actions/gestures
Umbral Mind Path:
1- Mesmerizing Shade: close range, will save partial, dazed and -1 attacks/checks/saves 1 round
2- Thoughts of Shadow: 1min/lvl, +4 to INT or WIS or CHA
3- Afraid of the Dark: med range, will save for half, 1d6+1 WIS damage
Cloak of Shadows Path:
1- Steel Shadows: 10min/lvl, +3 shield and +3 armor bonus to AC
2- Sight Eclipsed: 1rd/lvl, can attempt hide checks while being observed
Shadow Familiar/Improved Familiar
at 6th, it’ll be a Dark Weasel (+2 to REF saves)
at epic 2nd we’ll change to improved familiar and use the Gem Scarab from MMV
Dark Onyx Gem Scarab Familiar
N Small Magical Beast (Extraplanar)
Tremorsense 30ft, Listen +6, Spot +4
AC 19 (touch 11, flat 16) (+8 NA)
Speed 50, fly 70 avg, burrow 25, climb 20
Melee: BAB +2, Bite +2 1d4-1
SLA: Deathwatch 6/day (0th level spell from the Healer’s spell list, Mini 11)
Darkvision 60ft, Hide in Plain Sight, Resistance Cold 10, Superior Low Light Vision
Str 9, Dex 10, Con 16, Int 8, Wis 15, Cha 10
Alertness, Climb +9, Hide +7 (+15 in sand), Listen +6, Spot +4 (+10 racial on climb, and can always take 10), Improved Evasion, Share Spells, Empathic Link, Deliver Touch Spells, Speak with Master
Deathwatch gives you information on how close to death targets within a 30ft cone are. Each use is good for 10 minutes/lvl, and SLA’s are usually HD=CL (since familiar’s are special, that makes it for CL 6, or an hour at a time). Knowing which targets are close to death allows you to target them with your Arrow of Dusk to knock them out of the fight.

Spoiler: Explanations

So shadowcaster sucks, on that we can all agree. Class feature’s that are only usable a few times a day, and the rest of the time you have to be something else. Even the feats designed around the class either only grant an extra use 1-2x/day, making them poor choices to invest in as well for the most part. On top of that, any investment into this class destroys your BAB, so you’ll never be a frontline damage dealer either.
I’ve tried to use some of the available options to the shadowcaster, and enhance them, as well as to give myself more options for things to do in and out of combat. This starts out by having Beguiler be the base class for far more skill points, and arcane casting. Then following this with 3 ranks in the SI, finishing off with 2 ranks of Noctumancer, which progresses both the Beguiler’s arcane casting, as well as our mystery progression for Arcane CL of 6th, and Mystery 5th.
Umbral Sight stacks with our racial vision to grant Darkvision 90ft.
Precocious Apprentice allows us to hit 2nd level spells, as well as gives a potent attack vs enemy casters right from the get go. Shadow Cast grants you the ability to use all your spells/mysteries w/o provoking AoO’s as long as you aren’t completely surrounded, and Still Mystery makes all your mystery casting unidentifiable.
Shadow Familiar is explained above, but is most beneficial as you get full familiar advancement out of it instead of just partial. Martial Study shores up the abysmal REF save, while Shape Soulmeld stacks on bonus’s to Bluff and Hide. Favored Mystery bumps those 2 up to 3x day uses along with Extra Silence for the same, and Battle Caster gets us into Medium armor for more survivability. Darkstalker forces that astronomical spot vs our hide to be seen, and able sniper both grants a situational bonus to ranged attacks, as well as yet another bonus to hide.
Sight Obscured can be used with Bluff, Hide, Move Silently, Sleight of Hand and Tumble. The familiar shores up Arrows of Dusk making them more potent. We can buff any of our mental stats for what is most important in the moment thrice daily, get a boost to AC, or inflict WIS damage on a foe.
Our familiar has HiPs, and we nearly have it through our mystery. Total Hide score with everything up and activated comes out to +30. If you’re unfortunate enough to find yourself in melee combat, hopefully a +16 to your bluff check will help you to feint or otherwise cause a diversion. As a fun exercise in thought, your familiar is actually bigger than you, and uses and succeeds on a HiPs hide check, would your +30 modifier allow you to then use him as cover and hide behind him?
Noctumancer’s 2 class abilities present a bit of a problem, and for that we turn to UMD to help out. Dispel Magic is out of reach on our spell list, but with UMD we can attempt to use wands or scrolls (at a max of +8 to the check), or to do the same with mysteries that have been used with item creation feats to do the same. The second ability however can be used to burn off less awesome mysteries to outright counterspell, and in keeping that awesome shadowcaster tradition alive, once per day.

Spoiler: Sources

Complete Adventurer: Improved Diversion
Complete Arcane: Battle Caster, Practiced Spellcaster, Precocious Apprentice
Complete Scoundrel: Improved Familiar
Lords of Madness: Darkstalker
Magic of Incarnum: Lamia Belt, Shape Soulmeld
PHB2: Beguiler
Races of Stone: Extra Silence, Whisper Gnome
Races of the Wild: Able Sniper
Tome of Battle: Action Before Thought, Martial Study
Tome of Magic: Dark Creature Template, Feats (Favored Mystery, Shadow Cast, Shadow Familiar, Still Mystery), Fundamentals/Mysteries, Noctumancer, Shadowcaster