Just fiddling with the possibilities of creating new races from creatures in the Monster Manual.  (I'm so groggy and bored and unable to accomplish more meaningful things.)  I think the race itself is reasonably well balanced and translated from the monster entries, but I'm not so sure about the feats.

Stalking Step would actually stack with Defensive Mobility (a 1st level feat in the PHB which grants a +2 bonus to AC against opportunity attacks), giving the Dark One who has both a +4 bonus to AC against opportunity attacks, which is what the monsters have.  Shadowed Visage mimics the Dark One Emissary's ability to shift 1 square and gain concealment until the end of its next turn as a minor action, which would be too powerful for a player character.  Advantageous Stance just makes Stealth work like it did before it was changed in the errata (it no longer gives combat advantage).

Maybe next comes the Dark One racial paragon path, which could include the 11th level encounter attack power Dark Fog (creates a zone of darkness that blocks line of sight except against creatures with darkvision, sustain minor). Em, no.. that should probably be a daily.

DARK ONE "Native to the Shadowfell, dark ones are sly, murderous creatures." RACIAL TRAITS   Average Height: 3'6" - 4'0"   Average Weight: 60-75 lb.   Ability Scores: +2 Dexterity, +2 Wisdom   Size: Small   Speed: 6 squares   Vision: Darkvision   Languages: Common   Skill Bonuses: +2 Stealth, +2 Thievery   Dark Step: You can move 4 squares before taking a     penalty to Stealth checks.   Killing Dark: You can use killing dark as an encounter     power.   Shadow Origin: Your ancestors were native to the     Shadowfell, so you are considered a shadow creature     for the purpose of effects that relate to creature     origin. Killing Dark Dark One Racial Power When slain, a dark one explodes in a sprout of darkness. Encounter No Action Close burst 1 Trigger: You are reduced to 0 hit points or fewer Targets: Each enemy in the burst Attack: Dexterity vs. Reflex Hit: Target is blinded (save ends). Dark ones live among shadar-kai, often serving as agents, assassins, envoys, messengers, and spies. They can be found among various Underdark dwellers and prefer to live underground. They come to the world with sinister schemes, take and learn what they can, and slink back into the gloom. Play a dark one if you want . . .  * to use your shadow origins to enhance your abilities with dark powers.  * to be the halflings' more sinister counterpart  * to be a member of a race that favors the assassin, avenger, ranger, and rogue classes. Dark One Racial Feats: ADVANTAGEOUS STANCE     Prerequisite: Dark One     Benefit: You gain combat advantage against adjacent enemies with a successful Stealth check. SHADOWED VISAGE     Prerequisite: Dark One     Benefit: When you shift, as long as you do not end the movement adjacent to an enemy, you gain concealment until the start of your next turn. CREEPING DARK     Prerequisite: 11th level, Dark One     Benefit: The range of your killing dark racial encounter power increases to burst 2. At 21st level, it increases to burst 3. STALKING STEP     Prerequisite: 11th level, Dark One     Benefit: You gain a +2 feat bonus to AC against opportunity attacks when you move 4 squares or fewer. BLACKENED EYES     Prerequisite: 21st level, Dark One     Benefit: If you gain superior cover or total concealment, you become invisible until the end of your next turn.