So I realize I may be a little late to the party here, what with most of the discussion on the subject I've looked for apparently petering out a year or more ago and 5e on the way, but my friends and I only recently got into 4e. When we learned about the Assassin and the sadly-abandoned Shadow power source, we decided we wanted to run an all-Shadow party and went looking for homebrewed classes. While we found a few interesting and cool ideas some of us will likely use (particularly the excellent resource found here, before someone accuses us of not doing our research ), I wasn't 100% happy with any of the defender options others have come up with (most seem to be attempts to riff on the anti-paladin theme) and decided, why not make one ourselves? However, I have zero experience in homebrewing content for 4e, so I turn to you, the Internet, for help! I'm going to start with a core concept and work from there, hopefully with the input of you more experienced homebrewers out there to achieve a reasonable level of balance and fun flavor. I hope to eventually flesh this fledging class out until at least early Paragon tier, with a couple options for paragon paths and maybe some class-specific feats. Depending on how successful we are here and how happy my fellow players are with their characters after a couple play sessions, I may move on to other class roles later.

What I want to end up with is something that echoes the Assassin's style somewhat. A Dex-based defender that focuses on mobility, some stealth, and maybe a buildup towards a single "big" turn once every few rounds. Since Dex will be primary, I also kind of want to experiment with the idea of a ranged build option for the class too. Here's our initial idea, with commentary in italics.

The Veilguard

(Working name for now obviously, subject to change based on how the flavor of the class turns out.)

Role: Defender. You are an agile warrior who manipulates your foes with surprise, fear, and uncertainty to keep your allies from harm. You lean towards controller as a secondary role.
Power Source: Shadow. In a moment of desperation, you sacrificed a part of your soul for the power to protect what is most important to you.
Key Abilities: Dexterity, Charisma, Strength (Not 100% sure on what the 3rd key ability should be. I didn't want to exactly mirror the Assassin here, but it felt wierd for a defender to have neither Str nor Con as a key ability. Should I just make it Con?)

Armor Proficiencies: Cloth, leather, hide; light shield (Obviosly as a Dex-based class, they will want to stick to light armor.)
Weapon Proficiencies: Simple melee, military melee, simple ranged, light thrown, heavy thrown
Implements: Ki focuses (Per the assassin, I'm extrapolating. At the moment this is to allow for the ranged build I mentioned without having to use ranged weapons, but that might change.)
Bonus to Defense: +1 Fort, +1 Will?

Hit Points at 1st Level: 12 + Constitution score
Hit Points per Level Gained: 5 (According to Assassin, shadow classes are presumably squishy, so the Veilguard gets downgraded to leader HP. Class features may help make up for this.)
Healing Surges per Day: 8 +  Constitution modifier (Don't want them to be too easy to kill.)

Trained Skills: Stealth. From the class skills list below, choose three more trained skills at 1st level.
   Class Skills: Acrobatics (Dex), Athletics(Str), Bluff (Cha), Endurance (Con), Intimidate (Cha), Perception (Wis), Stealth (Dex), Streetwise (Cha), Thievery (Dex)

(Insert flavor here!)

Class Features:

Veilbond
Veilguards can manipulate the shadowy barrier between life and death to bind their fate to an enemy's. Whenever you hit an enemy with an attack, you can choose to mark that target. The mark lasts until the target ends its turn adjacent to you. Only one creature at a time may be marked in this way by you; marking another creature with your veilbond ends the previous mark, but if you mark other creatures with other powers, the target remains marked. At 16th level, you may have an additional creature marked at once with your veilbond. (Should I change it from marking on hit to an at-will that places the mark?)
In addition, choose one of the following features.

Umbral Veilbond: You gain the umbral bond power. Whenever a target marked by your veilbond makes an attack that doesn’t include you as a target, it takes a –2 penalty to attack rolls. If the attack misses, the targets of that attack gain concealment from the marked target until the end of its next turn, and the marked target takes damage equal to your Charisma modifier. 

Umbral Bond | Veilguard Feature
Sheets of shadow swirl around your foe, hiding your allies from its sight.
Encounter | Shadow 
Immediate Interrupt | Ranged 10
Trigger: A creature marked by you makes an attack that doesn't include you as a target.
Target: The attacking creature.
Effect: The target is blinded (save ends). It gets a penalty to saving throws against this effect equal to your Charisma modifier.

Constricting Veilbond: You gain the constricting bond power. Whenever a target marked by your veilbond makes an attack that doesn’t include you as a target, it takes a –2 penalty to attack rolls. If the attack misses, the marked target grants combat advantage until the end of its next turn and takes damage equal to your Strength modifier.

Constricting Bond | Veilguard Feature
Tendrils of shadow grip your foe, hindering its actions.
Encounter | Shadow 
Immediate Interrupt | Ranged 10
Trigger: A creature marked by you makes an attack that doesn't include you as a target.
Target: The attacking creature.
Effect: The target is restrained (save ends). It gets a penalty to saving throws against this effect equal to your Strength modifier.

(Definitely needs work, but I didn't want to go overboard. I was also thinking of allowing a ranged basic attack when the mark punishment is triggered instead of the current effects, but for that the class would need either a larger focus on ranged weapons or at least one at-will that can be used as a ranged basic attack.) 

Cloak of Shadows
Wrapped in darkness and illusion, veilguards are difficult to pin down, especially while moving. As long as you're wearing no armor or light armor, you get a +1 bonus to all defenses against opportunity attacks and ranged attacks; this bonus is increased to half your Dexterity modifier if you ended your last turn in a different space from where you started that turn. You also get a +2 bonus to saving throws to end effects that immobilize or slow you.

Slip the Veil
You gain the slip the veil power.

Slip the Veil | Veilguard Feature
As an enemy's clumsy attack whirrs by, you twist your veil around yourself to complete your escape.
At-Will | Shadow, Teleportation 
Immediate Reaction | Personal
Trigger: A creature misses you with an attack. 
Effect: You may teleport one square. 

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Progress Report: 
8/22: Implemented some of mattador666's suggestions. 

8/22: First draft. My top priority at the moment is making sure it has an interesting, functionally useful, and relatively unique marking mechanism that doesn't out- or under-perform too much compared to other defenders; the other features are less of a focus for now, but ideas for changes to them or entirely different ones are welcome as well. How's this initial attempt? Any feedback is greatly appreciated! ^_^

Reserved for more content later?

I've added some basic ideas for level 1 at-wills to give a general idea of what I had in mind, and added ranged weapons to the proficiency list for now; I'm not going to worry about altering the class features to support a ranged build until I get more feedback on what I've got so far.

We considered reflavoring existing classes, but decided doing things the hard way would be more fun. ^_^ By e-style I assume you mean Essentials style? Having a completed ranged defender to reference for ideas on how it might feasibly work would be very helpful, thanks!

Thanks! Took your advice on hide armor and Gravechill; should I try and fit a Str bonus into a different at-will, change the Cha bonus to Haunting Blow to static damage, or leave things as-is?

I kind of want to leave the HP at leader standard, since the Assassin had controller-standard HP; do you think it'll cripple the class too much?

Also, I think I'll just have a feat and/or PP that increases Slip the Veil's teleport and leave it at one for default.