The MCU introduced Thanos' ultra-competitive adoptive daughters Gamora and Nebula, as well as the intimidating Black Order, also known as the Children of Thanos. While these villains were the only survivors of Thanos' deadly "training," the Mad Titan did allow one more member to join in adulthood. Sadly, the Black Swan didn't make it into the MCU, despite being one of the most inherently powerful members of the Black Order.
Black Swan first appeared in 2013's New Avengers #1 by Jonathan Hickman and Steve Epting. Interestingly enough, Black Swan isn't an alien like the rest of the Black Order. Instead, Black Swan - then known as Yabbat Ummon Turru - was the lone survivor of alternate Earth-1365. She was raised in the Library of Worlds by three other Black Swans (who treat the name like a title) and raised to take on the mantle herself. She first came into conflict with the Avengers while trying to destroy the main Marvel Earth-616 as part of an effort to prolong her life in a collapsing Multiverse. Failing this, she was imprisoned in Wakanda's Necropolis.
Black Swan would ultimately take her place within the Black Order after Thanos reformed the group following the events of Secret Wars, but it took her a while to get there. While Thanos originally found her in Wakanda alongside the imprisoned Terrax, he initially turned down her plea to join his ranks. She wouldn't fight alongside Thanos until a rogue Namor, spurned by the Illuminati, released the mad Titan and his lieutenants - with Black Swan among him - to form his own Cabal.
However, after Thanos' supposed death, the new Black Order reformed to work at the behest of the Challenger in a battle against the Grandmaster's Lethal Legion. Black Swan and her colleagues most recently turned up in the pages of Star, as they sought the Reality Stone wielding by the title character. The miniseries saw Black Swan dealing with Star behind the backs of the Black Order, which ultimately backfires and leaves leader Corvus Glave trapped. Though this line of communication ultimately allows Star to cut a deal with the Black Order to ensure her own survival, Black Swan is reprimanded for her actions by the series' end.
It's Black Swan's underhanded sense of pragmatism which makes her such an unpredictable character. While she's just as ruthless as her Black Order comrades, she has shown hidden depths, such as when she is seen crying in her sleep, mourning her lost world while imprisoned in the Necropolis. While Black Swan didn't make it onto the big screen with the rest of the Black Order, an appearance in the MCU might not be far behind. After all, Doctor Strange and the Multiverse of Madness is poised to introduce Marvel's sprawling collection of alternate universes, probing Black Swan's natural playground. Given her religious hatred of magic-users, Black Swan could just be exactly the villain the MCU ordered.