The Black Order was a big disappointment in Avengers: Infinity War, one of the weakest links in a very well-balanced film. First appearing in 2013's New Avengers #8 created by Jonathan Hickman, Jerome Opeña, and Jim Cheung, the classic version of the group consists of Corvus Glaive, Black Swan, Ebony Maw, Supergiant, Proxima Midnight, and Black Dwarf. For the film, the team was streamlined to only four members: Glaive, Maw, Midnight, and Cull Obsidian (a dead ringer Black Dwarf)
Officially confirmed during last year's D23 Expo, the Black Order, re-purposed in the film as Children of Thanos, acts as the intergalactic villain's henchmen. Following their decimation of Xandar and the Asgardian ship that resulted with Thanos getting the Power and Space Stones, the evil foursome was the Mad Titan's emissaries to Earth, helping him track down the two Infinity Stones on the planet. Unfortunately, while the idea of them is good on paper and their presence in the film was pivotal to keep the plot moving, they were ultimately a let-down, especially in comparison to how fleshed out the Thanos himself was.
Design-wise, the Black Order looked menacing, sure; Marvel Studios stuck surprisingly close the foursome's look on the comics, and for the most part, they looked incredible. However, compared to their print inspiration, the team weren't quite as capable as they're originally made. In the source material, their respective skill sets almost rivaled their "father". On the silver screen, though, they ended up as one-and-done antagonists, meeting their end in Avengers: Infinity War with little fanfare. It was wasted potential on Marvel's part to use them as throwaway villains - in a movie that featured the rabid, brainless Outriders - especially when they could've done so much more than what they did.
Arguably the better-represented members of the Black Order were Ebony Maw and Proxima Midnight. Brilliantly played by Tom Vaughan-Lawlor, Maw was undoubtedly the leader of the pack, with enviable telekinesis and the most defined personality. But before fans even got to learn more about him and his powers, he met his death in the silliest way - thrown out of the Q-Ship, Aliens-style.
Proxima Midnight lived longer, but likewise felt underutilized. Played by Carrie Coon, the only female member of the group had some great fight sequences - especially against Black Widow - and strong banter with Black Panther and Captain America right before the battle in Wakanda. But she was absent for key stretches and likewise bit the dust unexpectedly (although did at least allow the 3-in-1 battle against Scarlet Witch, Okoye, and Widow).
The other two are where the Order's problems become more clear: Corvus Glaive and Cull Obsidian. Glaive, played by Michael James Shaw, is perhaps the worst translation of an Infinity War character to the big screen. In the comics, he's Thanos' right-hand man, something that was stripped away from him making him almost insignificant. Mostly known for his immortality stemming from the power of his glaive (which didn't really play a significant part either) as well as mastery in combat, the two times he was engaged in a fight, he was (relatively) easily taken out which doesn't check out with his supposed characterization.
Finally, the supposed muscle of the team was Terry Notary's Cull Obsidian. Not having any significant lines, the massive creature was little more than that. He put up a good fight against Bruce Banner who was aboard the Hulkbuster, but that felt like a missed opportunity to see the Hulk proper; as it is, he didn't appear as capable and strong as he's chalked up to be.
Narratively, the Black Order were necessary to keep the menace of Thanos somewhat restrained until the ending - something that helped make him Marvel's best villain - but that doesn't excuse their handling. For a film that hinged on the Mad Titan's relationship with other "children" Gamora and Nebula, they seemed to be little more than generic henchmen, with Avengers: Infinity War devoting little time to developing them. Then again, with so much stuff going on in the film, cutting corners in some parts were expected.