Black Order (Marvel Cinematic Universe)

The Black Order is an antagonistic faction serving as the secondary antagonists of the 2018 Marvel film Avengers: Infinty War. They are the adopted children and right-hand lieutenants of the notorious warlord Thanos in his quest to collect the Infinity Stones and remake the universe in his own image.

Guardians of the Galaxy
The Black Order didn't appear in the film, but they were mentioned by Nebula, who happens to be a member alongside Gamora.

Guardians of the Galaxy Vol. 2
The Black Order was mentioned again when Nebula voiced her anger towards Nebula due to the trauma she suffered at Thanos when they were training alongside the other members of the Black Order, saying that she only wanted Gamora to act like a sister instead of focusing on her intents to win their sparring battles against each other.

Avengers: Infinty War
The Black Order finally made their appearance in the film, where they are first seen helping their father acquire the Power Stone (the Orb) by invading and destroying the planet Xandar. They later pilot their giant ship as they spotted another ship containing Thor, Loki, Hulk and the Asgaradian survivors following the destruction of Asgard. Being aware that Loki has gotten hold of the Space Stone (the Tesseract), Thanos gets the Black Order to board the Asgardians' ship and attack the occupants. During the siege, Hulk was sent back to Earth to warn of Thanos' arrival by Hemidall, who ends up being killed by Glaive. The Black Order then helps their father into coercing Loki into surrendering the Tesseract, just as Thanos proceeds to kill Loki and use the Orb to destroy the Asgardians' ship, killing all of the Asgardians in the process. Thanos and his children then depart away from the scene in their ship, leaving a helpless Thor to drift in space before he was rescued by the Guardians of the Galaxy.

With the Orb and Tesseract in his possession, Thanos gets the Black Order to split up and search for the other Stones on Earth. As Thanos heads alone to force the Collector into giving him the Reality Stone (the Aether), Maw tracked down the Hulk to New York City, where he stole the Time Stone (Eye of Agamotto) by taking its occupant Doctor Strange as their captive, intending to deliver him to Thanos at the planet Titan. In the meantime, Midnight and Glaive head over to Scotland, where they ambushed Scarlet Witch and Vision (as the latter holds the Mind Stone on his forehead). However, Captain America, Black Widow and Falcon arrived to the rescue by bringing Scarlet Witch and Vision to Avenger HQ, where they plan to go to Wakanda to destroy the Mind Stone.

After killing the Collector to get hold of the Aether, Thanos kidnaps Gamora and coerces her into giving the location of the Soul Stone by threatening to torture Nebula. Gamora then gives the location: a planet called Vomrir, where the Red Skull serves as the stone's guardian. Thanos would later reluctantly kill Gamora in order to take possession of the Soul Stone per Red Skull's suggestion.

In the meantime, Iron Man and Spiderman manage to head over to Titan, where they killed Maw to save Strange and the Eye of Agamotto, just as Nebula contacts Thor and the other Guardians to meet up with her. However, Thanos arrived to the scene as well and overwhelmed the group before coercing Strange into giving up the Eye of Agamotto.

With five Infinity Stones in his possession, Thanos directed Obsidian, Midnight and Glaive to assemble a massive army of Outriders to attack Wakanda in order to obtain the Mind Stone from Vision. This forces several of the Avengers and the Wakandan warriors to lay down a defense against the Black Order and the Outriders. Despite being outnumbered by the massive armies, the Avengers and Wakandan warriors were able to turn the tide to destroy the Outriders and kill Glaive, Midnight and Obsidian for good, just as Thor arrived with Rocket Raccoon and Groot with a new weapon to finish off an arriving Thanos while Scarlet Witch tries to destroy the Mind Stone per Vision's instructions.

Unfortunately, despite the loss of the Black Order and the Outriders, Thanos was able to collect the Mind Stone before killing Vision. Showing no concern over the loss of his adopted children and his army, Thanos used the power of all six Infinity Stones to wipe out half of the universe's populace (including several Avengers and the majority of the Guardians). This left the remaining heroes to mope in defeat while a triumphant Thanos oversees a sunrise from another planet, satisfied that he finally achieved his goal.