The
Quickling is one of the smallest creatures in the game, and gets some really fun abilities, as well as fitting into the theme that Fey Queens are not very nice.
The best thing about the art in Volo's is the fact that the Quickling is standing on a water lilly, a detail that I didn't notice until coming back to write this entry. It's a fun way of showing both the size and speed of the Quickling, while its body features both an alien looking head, an improbably thin waist, and well-developed calf muscles, all of which seem eminently appropriate.
The Quicklings were originally a race of lazy Fey, who managed to offend the Queen of Air and Darkness by repeated tardiness. I'm wondering just how many of them she summoned at once, for them to turn into an entire race of Fey thereafter. Anyway, they turned from lazy into Quicklings, but got the lifespan to match. They seem to have been not very nice before this transformation, but afterwards are definitely Up To No Good, with a Chaotic Evil alignment and a fluff description that feels the need to say 'they don't actually murder people' in bold letters.
A lot of space here is devoted to explaining how slow everything looks to the Quickling, which is probably more news to anyone who has not seen
The Flash or
Age of Ultron. Here we learn that the Quicklings are more consistent than comic book speedsters though, preferring to remain always at superspeed. They make use of this power by annoying and pranking people, rather than cornering the market on miniature goods manufacture which would have been the logical thing to do. On the plus side, "Adventurers raid the evil and very small clockmaking factory" is probably not the most exciting module ever.
As a side effect of spending all that space on speed, the fluff description doesn't really give a good idea of what you're meant to do with the Quicklings. We don't hear anything about their society or family networks, how they interact with other Fey, or what Quicklings actually want out of life. You're on your own there, apparently; and these guys seem a lot harder to use than other Fey like Darklings or Korreds.
However, their statblock is pretty great, so let's look at that. With CR 1, the Quicklings are surprisingly low level, but look really dangerous opponents despite that. AC of 16, disadvantage for enemy attack rolls against it, due to its great speed, and the
Evasion ability of a Rogue combine to make the Quickling's 10 HP tough to actually get at. This is on top of a rather grotesque 120ft of movement a turn, and huge bonuses to Stealth and Sleight of Hand, mean that we should probably be happy that they didn't give this creature
Disengage as a bonus action; opportunity attacks are likely to be desperately needed by your players. Like the Nilbog, a low level group that is unlucky on the dice is likely to get ground into paste by one of these. On top of that, the Quickling does an impressive three attacks a turn, which is great for the imagery of it rapidly stabbing people, but also means that it is going to have a consistent damage output, especially with 1d4+6 damage on each hit, which gives a narrow 7-10 range of results.
The Quicklings are a race that I'd be tempted to hold off on until the characters are level 3-4, and then use in small swarms to enrage and pinprick them. They should also work well with other Fey, especially since they are well placed to pounce on anyone that gets Restrained by a Korred or Hag. However, we don't really get a lot of explanation of what story these guys go with, which is a shame.