This is Makeup and not CGI - Stranger Things BTS Video
Vecna invades his victims' thoughts and preys on latent guilt before twisting their bodies into human pretzels, inspired by legendary villains like Freddy Krueger and named after the Dungeons & Dragons spellcasting demon. While Bower's baritone voice may seem like Freddy Krueger, the character is really played by Robert Englund (who plays Victor Creel in the series). He stalked Chrissy, Fred, and Max while wearing pounds of prosthetics, in stark contrast to his role as the "Friendly Orderly."
Stranger Things' cast reacted to the scene from episode 4, "Dear Billy," in which Max escapes Vecna in his mind lair owing to Kate Bush's "Running Up That Hill," in a recent interview during Netflix's Geeked Week. Following Bower's reaction, viewers are treated to behind-the-scenes footage of him in the villain's realistic outfit.