Okay, the Kobold Assasin is "done". I have a few comments after the stat block.
Kobold Assasin Level 6 Lurker
Small Natural Humanoid (reptile) XP 250
Initiative +12
Senses Perception +11, darkvision
HP 56;
Bloodied 28
AC 20;
Fortitude 17,
Reflex 20,
Will 18
Resist 5 poison
Speed 6
Poisoned Dagger (Standard; at-will) ♦ weapon, poison
Attack +11 vs. AC; 1d10+4 poison damage
Study weakness (Standard; at-will)
The Kobold Assasin designates a target that it studies for weaknesses. Each time this power is used, it increases the damage of the next attack the Kobold Assasin makes against the designated target by 1d10+4 (to a maximum of 2d10+8). If a new target is chosen, the previous increases in damage is lost. This power requires line of sight to the target.
Run-Hit-Run (Standard; at-will)
The Kobold Assasin moves up to its speed and makes one melee basic attack at any point during that movement. The Kobold Assasin doesn't provoke opportunity attacks from the target during this movement.
Hide in the Shadows (Move; recharge



)
If the Kobold Assasin moves at least 4 squares, it gains concealment until it makes an attack.
Shifty (Minor; at-will)
The Kobold Assasin shifts 1 square as a minor action.
Trap Sense
The Kobold Assasin gains a +2 bonus to all defences against traps.
Alignment Evil
Languages common, draconic
Skills Stealth +13
Str: 14 (+5)
Dex: 20 (+8)
Wis: 16 (+6)
Con: 14 (+5)
Int: 14 (+5)
Cha: 16 (+6)
Equipment dagger, leather armor
I remember reading a blog post about Lurkers not acting as they were described in the DMG: "They usually deliver one devastating attack every few rounds, while concentrating on defense in between".
I tried to incorporate that concept.
This monster does well coupled with some monsters that last a long time and can keep the players occupied so they wont go looking for the Kobold Assasins.
I am not sure about some of this (no PHB nearby), so feedback is very welcome. My idea was that the Kobold Assasin hides and studies a target (most likely a cloth wearer) with "study weakness", then hits it with "run-hit-run" and moves away again, activating "hide in the shadows" which grants it a stealth roll. It then studies the same or another target for two rounds, giving "hide in the shadows" plenty of time to recharge.
Also, I am wondering this:
Would it be able to get combat advantage against an opponent if it is stealthed (and has concealment), but has to run out "in the open" to hit the target? This means that it would actually pay off to wait for the 3d10+12 damage, so you can also attack with CA.
Would it be an idea to have the extra damage last until the Kobold Assasin makes a successful attack? Missing the first two attacks would mean the combat was basically over before you got a new one in.
Edit: If anyone was wondering, the inspiration is from the 3.5 prestige class: Assassin.
And looking at it, the damage might be a little high. I am not sure it would be fair to use two of these in the same encounter.