Dark Ones are small humanoids from the Shadowfell, echoing the world’s halflings in the same way Shadar-Kai (which we’ll eventually get to) echo its humans. They’re halfling-sized with purplish skin, pronounced noses, pupiless eyes and sharp teeth. I think the one pictured in the book also has hooves and is wearing boots over them, which is somewhere between cute and funny. Overall, though, the Shadowfell aesthetic is definitely there.
Since they’re Unaligned, I imagine there are entire communities of Dark One civilians out there leading peaceful existences alongside Shadar-Kai villagers and the like. Their culture probably bears a vague resemblance to that of halflings but is much gloomier. This all speculation on my part, though, as the book deals with the sort of Dark One PCs are likely to fight.
Dark Ones have at least as much talent for skulduggery as haflings, so the ones PCs meet as antagonists will likely be up to something shady - espionage, theft, assassination, and so on. They often work for shadar-kai, intelligent undead, and Underdark dwellers. Even then you could still make them enigmatic spy types that might ally with the PCs if their goals happen to match.
We have two varieties here, both Unaligned Small Shadow Humanoids with Speed 6 and darkvision. They’re trained in Stealth and Thievery.
The Dark Creeper is a Level 4 Skirmisher with 54 HP. It carries 5 daggers hidden in its black garments, and uses them to attack both in melee and at range. It deals extra damage against targets that grant it combat advantage, and has a Dark Step at-will movement power that allows it to move up to 4 squares while gaining a +4 bonus to AC against opportunity attacks, and to gain combat advantage against anyone it ends up adjacent to.
It clearly fights by zipping around the battlefield as a cloud of shadowy smoke and stabbing fools. It won’t avoid OAs every time, but will still avoid enough to be a nuisance. When the Dark Creeper dies, it “explodes in a spout of darkness”, automatically blinding anyone (save ends) in a close burst 1.
The Dark Stalker is a Level 10 Lurker with 81 HP. It wears the same black garments and also carries a bunch of daggers, but prefers a scimitar for close-in work. It has the same abilities as the Dark Creeper, plus a few extras:
Dark Fog is an encounter power that creates a zone of darkens in an Area Burst 4 within 10 squares of the stalker. This zone lasts a turn but can be sustained with a minor action. It blocks line of sight for anyone who doesn’t have darkvision.
Finally, the dark stalker can become invisible for a turn with a minor action, a power which recharges on a 3-6.
While the Dark Stalker can use Dark Fog and Invisibility to hide a better than the Dark Creeper, the fact that it has stronger versions of all the creeper’s powers strongly indicates to me it should be a skirmisher, and not a lurker. This would make it a better able to survive the thick of the action.
The sample encounter is level 4: 3 dark creepers, 2 shadowunter bats, and 1 deathlock wight. I would strongly suggesting giving the creeper trio the ability to drop a single zone of darkness like the stalker’s as a joint stunt. That would give both the bats and the creepers some extra kick.
I like these! Their lore is kinda sparse but suggests several interesting plot hooks. The signature traits for making custom dark ones are their Small size, the Killing Dark power that makes them explode into ink, and Dark Step. It’s easy to make either these existing dark ones or your custom models of a leve that makes them likely to hang out with shadar-kai.