Just because Thanos is finally searching for the Infinity Stones himself doesn't mean he doesn't have help. Along with his army of Outriders, Thanos will be assisted in Avengers: Infinity War by The Black Order, whose members, like Gamora and Nebula, were taken from their homes at a young age and raised to be assassins for the Mad Titan. The Black Order have only been around for five years in the Marvel Comics universe, but when it came to crafting Infinity War's story, it was necessary to have these minions involved so that they could serve as Thanos' proxies while he was occupied elsewhere. Co-director Joe Russo and co-writer Christopher Markus explained:
Joe Russo: Well, there are a lot of characters, and you need those characters to have conflict with someone, and Thanos can't be in all places at all times in the story. And our characters are spread around the cosmos.
Christopher Markus: Same scene over and over again. Trying to avoid the same scene over and over again, so that he's got a mission. He sends people out to help him, and there are complications as a result.
The Marvel Cinematic Universe has been building to Thanos's reign of terror ever since he smirked at the camera in The Avengers mid-credits scene, and now we'll finally see why he's to be feared in Avengers: Infinity War. That being said, even with his considerable natural abilities and the powers that each Infinity Stone grants him, Thanos can't be everywhere at once, and the movie would likely become boring if we only saw each Marvel hero one after the other trying to take down the Mad Titan in just one location. Hence why, as Joe Russo and Christopher Markus told CinemaBlend's own Eric Eisenberg, the Black Order had to be brought in to serve on Thanos' behalf. They can help him retrieve the Infinity Stones, and that subsequently helps build up to the movie's climactic battle. Markus' writing partner, Stephen McFeely, added:
Stephen McFeely: You want to build up to the big climax. If he's visited Earth five times to go, 'I need that stone, I need that stone,' you wind up having the same fight five times.
With the exception of Supergiant, who's sitting out the MCU, this Black Order will have the same lineup as in the comics, with Terry Notary playing Cull Obsidian, Tom Vaughan-Lawlor playing Ebony Maw, Carrie Coon playing Proxima Midnight and Michael James Shaw playing Corvus Glaive. Make no mistake, Thanos is the main threat in Avengers: Infinity War, but from the footage that's been released so far, the Black Order are formidable in their own way; particularly Ebony Maw, who has been shown torturing Doctor Strange. But when push comes to shove, the MCU heroes will have to fight their hardest to make sure that Thanos doesn't wipe out half the life in the universe with the Infinity Stones.