Demogorgons are predatory creatures originating from the Upside Down, an alternate dimension. Demogorgons obey the malicious Mind Flayer, and make up part of his hive mind. Under his influence, Demogorgons are murderous, violent and have limited intelligence.
In 1983, a fully grown Demogorgon set foot in Hawkins, Indiana after a gateway to the Upside Down was accidentally opened. This Demogorgon was eventually defeated and destroyed, but not before it could inflict damage upon the human world. Will Byers, a rescued victim of the Demogorgon, became a vessel for the Mind Flayer. Somehow, Will's possession allowed the Mind Flayer to unleash a swarm of adolescent Demogorgons upon Hawkins.