In "Stranger Things," the boys call the giant shadow monster from the Upside Down the Mind Flayer, borrowing the name from Dungeons & Dragons. Dustin, Mike, Lucas, and Will use the name because the entity resembles the powerful, mysterious monster from their tabletop game.
The Mind Flayer is introduced in season 2 as a vast, intelligent presence connected to the Upside Down's creatures through a hive mind. It invades Will's mind, uses visions to influence him, and directs the Demodogs and underground tunnels beneath Hawkins. The group eventually closes the gate, cutting off its connection to their world.
The name is a deliberate Dungeons & Dragons reference, but the television creature is not identical to the game's monster. In D&D, a mind flayer is typically a humanoid creature with tentacles around its mouth and powerful psychic abilities. "Stranger Things" instead presents a towering, spider-like shadow entity. Vecna is a separate antagonist, while the Demogorgon is another Upside Down creature.