The large shadow monster that threatens Hawkins in season 2 of "Stranger Things" is the Mind Flayer.
The entity is introduced through Will Byers's terrifying visions of a colossal, spider-like shadow looming over Hawkins. It comes from the Upside Down, a parallel dimension linked to the town, and operates through a hive-mind connection with the creatures there. The Mind Flayer eventually implants part of itself in Will, allowing it to spy on the real world and direct the adolescent Demogorgons, or Demodogs, beneath Hawkins.
The season's central conflict is the group's attempt to sever the Mind Flayer's connection to Will and close the gate at Hawkins National Laboratory. Heat helps drive the entity from Will, while Eleven uses her powers to close the gate and cut off its access to Hawkins.
Viewers sometimes confuse the Mind Flayer with Vecna or the Demogorgon. The Demogorgon is the primary monster of season 1, while Vecna is a separate major antagonist revealed later. In season 2, the Mind Flayer is the enormous shadow presence directing the invasion.