I wonder why the Fey didn't get lumped together in this book. I mean, Dragons and Demons each get one collective entry. Perhaps the thinking was that the Fey are all so different that they needed to be separated out to make them understandable. Regardless, we now meet the
Darklings, strong contenders for edgiest guys in the Feywild.
The art in Volo's is pretty generic. The two Darklings are just sort of standing there, looking menacing. I think that they would have really benefitted from a background, perhaps a dark corner with a nearly-obscured painting behind them, to highlight their themes.
The Darklings have a pretty interesting story. Like many Fey, they got cursed by another more powerful Fey, in this case the Summer Queen. Is that different from Queen Titania? Who knows. Anyway, they got cursed to age prematurely under light, and so they keep themselves wrapped up to try and avoid it as much as possible. The way that this works is that they actually store the energy of the light, and upon death they explode in a little sunburst; which means that when one is close to death, the others would have to shun it for fear of dying themselves, which is a rather sad thought. Cursed to die alone!
The other sad part of this curse is that they have a love of art, and thus are driven to take the risk of an open flame just to peek at a painting. That seems like a really interesting plot hook; an art gallery hires heroes to find out who keeps breaking into the place but not stealing anything, and they meet one of these guys. I'd be strongly tempted to use such a Darkling as a PC ally, their 'man on the inside' to guide them into Fey society as it were. However, the flavour description also makes it clear that these guys operate as thieves and assassins, from caverns beneath "the towns of other species". Does that mean other Fey, or Humans and the like? I'm guessing that means the latter, despite there being basically no mention of Fey in any of the 5e books thus far. Either way, the Darklings are clearly not unambiguously good guys, and have the dreaded Chaotic Neutral alignment, so you've got a lot of room to use them as you see fit.
They can make for interesting bad guys, a more magical Assassin's Guild sort of thing, even as you can play up their tragic story at the same time as a means to building rapport between the two sides. You could try for a bad guys to good guys arc, having the players initially fight against the Darklings, and then be driven by pity, convenience, and a greater threat to work
with them against other more powerful and malevolent Fey. The end result would be low level bad guys who feel
very different from Goblins or Orcs.
Combat wise, the Darklings are like Rogues. They have good stealth scores, not much health, and they are afflicted by
Sunlight Sensitivity. They have bonus damage when they have advantage, but not just for mobbing people. Happily, the CR 2 Elder version can cast
Darkness, which will allow the Darklings to use their
Blindsight to gain said advantage. Nasty. When they die, they let out a bright flash of light, which can inflict the Blinded condition, even better for the Darklings who can (rather oddly) ignore it and press the attack using their Blindsight.
Not the most complicated of critters, but distinctive and interesting, with a really poignant backstory. Definitely guys that I'm eager to use.