In the 2012 film The Avengers, the main villain is Loki. Played by Tom Hiddleston, Loki leads the film’s central attack on Earth and serves as the Avengers’ primary on-screen antagonist.
Loki arrives with the Tesseract, an immensely powerful energy source containing the Space Stone, and uses it to open a portal above New York City. Through that portal, the Chitauri invade Earth. Loki’s goal is to conquer the planet and prove himself worthy of ruling it, continuing the character’s rivalry with his adopted brother, Thor.
A common mix-up is Thanos, who appears in the mid-credits scene and is revealed as the force behind Loki’s alliance with the Chitauri. Thanos is an important hidden power in the story, but Loki is the villain the Avengers directly confront throughout the film. Red Skull belongs primarily to Captain America: The First Avenger, while Ultron is the antagonist of the 2015 sequel Avengers: Age of Ultron.
The film was Marvel’s first major ensemble team-up, bringing Iron Man, Captain America, Thor, Hulk, Black Widow, and Hawkeye together against Loki’s invasion.