Vecna, a scary new villain who is murdering Hawkins teenagers, is introduced in Stranger Things season 4 volume 1. Every season of Stranger Things introduces a new monster, but for season 4, instead of a mindless creature like the Demogorgon or a hideous intelligence like the Mind-Flayer, the show introduces a powerful person who has been living in the Upside Down. The Hawkins kids call this creature "Vecna," and it manifests itself in the actual world through terrifying visions that they compare to a curse.
Stranger Things season 4 episode 7 eventually reveals the truth about Vecna. The hideous monster is revealed to be Number One, Dr. Brenner's first subject at Hawkins Lab, thanks to a series of flashbacks. Number One possessed all of Eleven's abilities, but he was successfully controlled by Brenner for years; he then persuaded Eleven into releasing him, conducting a bloodbath at Hawkins that was only stopped when Eleven herself confronted him. Their fight was Stranger Things' true inciting act, with Eleven triumphing but blowing through a portal between realities, throwing Number One into the Upside Down.
Vecna has merged with the hive consciousness of the Upside Down. Although the Hawkins kids believe he now serves as the Mind-"five-star Flayer's general," it's unclear whether this is the truth; the Mind-Flayer is noticeably gone from the Upside Down's skyline in Stranger Things season 4, suggesting Vecna has taken its position as the hive mind's ruler. Whatever the situation may be, Vecna's curse appears to be powerful - and crucial to the plot of Stranger Things season 4.