Do not underestimate the power of this feat. Although it’s online a little early here, it can make all the difference against most anti-stealth measures people will take.

Shock and Awe is an amazing spell from the Spell Compendium, but it’s normally pretty limited in when it can be employed – surprise rounds only. It’s a great fit for sneaky beguilers like yourself – cast it on whoever will raise the alarm or who can make or break an upcoming fight. Improved Diversion can keep you hidden while you cast – it speeds up your ability to create diversions to the point where you can cast spells and shout “look behind you, a three-headed monkey!” in the same round. You’re unfortunately limited to a 5’ step in hiding, but with Darkstalker and the right use of Silent Image, that will do.

Fog Cloud and Invisibility start to compete with Silent Image for providing a hiding place in combat. Bump Int at this level.

This feat will become much more useful later, but it’s good to have a way to mask your vocal components.

This feat allows you to move and cast a spell as a single standard action. Improved Diversion and the Beguiler’s Surprise Casting allow you to bluff to feint or create a diversion as a move action. You can, in a single round, vanish from the sight any foe without the Sense Motive or Spot to match your Bluff and Hide checks; casting a Fog Cloud or Invisibility and moving is hard to counter when you’ve got Darkstalker.

This is a lovely save-or-up-yours that grapples for an extended period of time against foes who fail their save. Action advantage and denial of Dex? Hell yes!

Improved Invisibility appears, allowing you to be more aggressive. Boost your Int score again and start putting ranks into UMD. Freedom of Movement also appears, protecting you from sticky situations.

And now using Silent Spell no longer costs you bigger actions, freeing you up to move or Bluff.

Not quite as handy as Silent Spell, but hey, when you need to cast a Still Freedom of Movement, you’ll appreciate it.

A magnificent spell; it entangles enemies and lets you hide while observed in its effect. Probably hands down the best spell in your arsenal for when you gneed to both make life difficult for your enemies and disappear GNOW.

This PHB2 feat gives you two benefits while invisible. One, enemies take a -5 penalty to Listen checks to locate you. Two, you can make an Attack of Opportunity against any foe who attacks the square you occupy (though it tells them where you are and removes any miss chance). The first benefit is useful now; the second not quite yet.

In addition to that nice little DC boost, two things are worth gnoting. First, you gain access to Project Image at this level, which is just unfair in a beguiler’s hands. See how Project Image combines with other illusions. You can create more decoys by casting Greater Mirror Image through your projected image, change its appearance with Disguise Self and make the enemies believe there’s a traitor in their ranks, or just enjoy another invisible point of origin for your spells by casting Greater Invisibility on it. Second, if you’ve been boosting UMD since level 8, by this point you should have a fairly good amount of ranks in there (especially since we haven’t had to spend skill points on Spellcraft for five levels). Get a Circlet of Persuasion and a few useful wands to deal with enemies that are immune to mind-affecting effects.

Evasive Reflexes allows us to take a non-provoking 5’ step in place of an AoO. Remember that second benefit to Spectral Skirmisher? Yes, that's right; they attack the square you’re in, and you’re suddenly gnot in it anymore. This is the level by which Serpent Armor becomes very handy, but only wear it if you can get that enchanted with Greater Shadow and Silent Moves enhancements; the primary function of armor for us is to boost our Hide and Move Silently checks. Also take any enhancement that would remove ACP. We don’t care about our Armor Class; a guy who can’t be targeted doesn’t gneed to worry about being hit. Alternatively, with your UMD you can use an Eternal Wand of Heroics to supply yourself with Combat Reflexes, and forget the Serpent Armor.
As a second point, Simulacrum can cause… problems if the DM isn’t prepared. Not only can it produce decoys or body doubles (including duplicates of yourself – see everything up there with Project Image? Now double it and shave off a few levels from the double), it can also duplicate NPCs. Expect a clever player to be able to do great things with this spell.

You acquire Moment of Prescience at this level. If you’ve got a Ring of Evasion, this is the spell that will save your bacon when they stop trying to look for you and start blasting the area with explosive magic.

Combat Reflexes is the last piece of your “gnever touch me” engine. With it, you can use Evasive Reflexes more than once per round, allowing you to dodge multiple attempts to attack your square. If your GM allows you to get Serpent Armor modified with the appropriate enhancements, or if you have an Eternal Wand of Heroics, then you do not need to take Combat Reflexes. In such a case, take the Extraordinary Concentration feat from Complete Adventurer. This lets you concentrate on a spell (like Scintillating Pattern or Major Image) as a move action. Remember Mobile Spellcasting? You can move, cast, and concentrate all in the same round, and that doesn’t even affect your swift action.

Excuse me while I add about half of the Conjuration school to my beguiler spell list.