The main humanoid antagonist introduced in season 4 of "Stranger Things" is Vecna. He is a murderous supernatural entity who hunts people burdened by trauma and attacks them through visions, nightmares, and telekinetic violence.
Season 4 reveals that Vecna was once Henry Creel, the son of Victor Creel. Henry possessed powerful psychic abilities and developed a deeply hostile view of humanity. After being taken to Hawkins National Laboratory, he became Subject One and was later transformed into Vecna after a confrontation with Eleven.
Vecna operates from the Upside Down and is connected to the supernatural threats seen earlier in the series. This revelation makes him more than a new monster: he is tied to the history of Hawkins and the origins of the Mind Flayer's terrifying influence. The Demogorgon and Mind Flayer are major antagonists, but neither is the humanoid villain newly introduced in season 4.