The classic X-Men villain known as the Adversary has been revealed as the surprise antagonist in the current Phoenix Song: Echo limited series. Although his identity was not revealed in the October 20's Phoenix Song: Echo #1, the Adversary's involvement was confirmed in Marvel Comics' January 2022 solicitations in describing the book's fourth issue.
Originally created in 1984 by Chris Claremont and John Romita Jr. for Uncanny X-Men #188, the Adversary is a being from another dimension who came to Earth with a plan to destroy and remake this universe more to his liking.
Although best known as an X-Men villain, in his backstory it's revealed he's been a constant thorn in the side of the Cheyenne people - whom the X-Men Forge. According to the storyline, Forge was actually trained from birth to battle the Adversary - and has done so both in Uncanny X-Men's 'Fall of the Mutants' story as well as later in an arc of X-Factor.
The newly-minted Phoenix, Echo, is actually of Cheyenne and Latino heritage, and in Phoenix Song: Echo #1 she began seeking out more of her Native American roots - including what at the time appeared to be an unrelated meeting with Forge over her newfound Phoenix Force powers.
The problem is, with Echo now in line to face the Adversary over the course of this five-issue series, she'll need those Phoenix powers - but according to the description of January 2022's Phoenix Song: Echo #4, the Phoenix Force will leave Echo by the time that issue takes place.
The story continues in Phoenix Song: Echo #2 on November 24.
The Adversary nearly made Newsarama's list of best X-Men villains.