Basically the “bait and switch” trope, but with the villain’s identity. The story sets up one person to be the main antagonist, only to reveal that the REAL main antagonist was just biding their time. Maybe they had a plan they wanted to get into motion first, maybe they weren’t involved in the story yet, or maybe they’re just starting to become villainous. A twist villain can also fall into this category, provided that, in their twist, they supplant another lesser villain to become the main threat of the story.
Characters in this trope get bonus points if they beat or kill the previously assumed main villain in a fight.
Examples (SPOILERS BELOW):
Kilmonger from “Black Panther” (2018): the example that inspired this post. Both “Age of Ultron” and the first act of this film set up Ulysses Klaue, the delightfully evil Afrikaner arms dealer played by Andy Serkis, to be the main antagonist. But after evading our heroes, one of his henchmen abruptly kills him, right after revealing his Wakandan heritage. That henchman is Erik “Kilmonger” Stevens, and his evil ambitions far exceed his former boss’s.
Azula from season 2 onward of “Avatar: The Last Airbender” (2006): throughout season 1, the exiled Fire Nation prince Zuko is the most persistent threat to Team Avatar. At the start of season 2, his sister Azula travels to the Earth Kingdom, initially seeming like she wants bring Zuko back into the family. But once her true intentions are revealed, she attacks him, places a bounty on him, and makes his exile seemingly permanent. While Zuko remains in the story and still has moments of antagonism on his path to redemption, Azula becomes a much bigger active threat to Team Avatar than Zuko ever was.
Tetsuo from “Akira” (1988): at first, the main antagonist appears to be Colonel Shikishima. He’s responsible for public order in the corrupt cyberpunk hellscape of Neo-Tokyo, which he maintains with extreme force, and orders the detention of Tetsuo, the best friend of our main protagonist Kaneda. However, Tetsuo was detained for a reason - he starts developing godlike ESP powers, rampages trough the hospital he’s being detained at, confronts Shikishima and the other ESPers, and then leaves to find the remains of Akira, tearing a path of destruction and violence through Neo Tokyo as he goes. Shikishima tries to stop this, throwing everything he as at Tetsuo before eventually being indirectly killed by him.
Tighten from “Megamind” (2010): a rare example of this trope being done with a villain protagonist. After Megamind seemingly kills Metro Man and gets everything he ever wanted, he grows bored and decides to recreate Metro Man so he can have a nemesis again. The person he builds up as this new hero, Hal Stevens Hal Stewart instead acts upon his worst aspects once gaining godlike powers. Donning the name “Tighten,” he demonstrates himself to be far more openly malicious and evil than Megamind ever was, and Megamind completes his heroic turn by stopping Hal.
EDIT: slight mistake: Shikishima doesn’t die. I thought he got killed when Tetso entered his final form, but I just forgot that the ESPers teleport him away. Whoops.
I’ve seen “Akira” tons of times in both English and Japanese. I honestly have no idea how I made this mistake.
EDIT 2: Hal Stewart, not Hal Stevens. God, I need to fix my sleep cycle.