The Mind Flayer

The Mind Flayer is an entity originating from the Upside Down, an alternate dimension existing in parallel to the human world. The Mind Flayer is gigantic, looming at least 50 stories tall, and sports a distinctive spider-like appearance. The Mind Flayer could wield supreme control of the Upside Down via a psychic link, with Demogorgons, vines, and demobats forming a shared hive mind; for a period in 1985, possessed humans known as "The Flayed"would also comprise part of the hive mind. In a sense, the entirety of the Upside Down was like one gigantic organism, with all its contents and agents connected both physically and mentally via the Mind Flayer.
The earliest form of the Mind Flayer was discovered by Henry Creel between 1979 and 1983, following Henry's banishment to the Upside Down. This version of the Mind Flayer was a storm-like mass of shadowy particles, or mist; it is not known if the mist possessed its own independent will, or consciousness. Henry used his powers to reconstitute the mist into a form that satisfied him: a giant, spider-like entity with a flame-shaped head, not unlike a similar creature he had fantasized about and illustrated as a child. 
Vecna first actively attempted to take over the world in 1984. Acting through the Mind Flayer, he possessed Will Byers and an army of adolescent Demogorgons, or "Demodogs". Eleven interfered by closing the interdimensional gate she previously opened at Hawkins Lab, severing the psychic link between the worlds and killing the Demodogs in the process. As Eleven closed the Gate, Will's friends and family worked to force a piece of the Mind Flayer out of his body, which in turn became dormant after the Gate's closure.
The isolated piece of the Mind Flayer remained dormant at Brimborn Steel Works until 1985, when a newly opened Gate reestablished the psychic link to Vecna. Via this piece, Vecna began kidnapping and possessing multiple humans and rats, feeding them chemicals and melting their bodies into puddles of biomass. This biomass was then combined to create a proxy form, through which Vecna could directly act in the human world; like the Mind Flayer, this proxy form also resembled a spider, and had the same flame-shaped head. However, this proxy form was rendered useless and inert after the new Gate was shut, and the psychic link disabled once again. 
In the same time period, the Soviet Union somehow captured a portion of the Mind Flayer's particles and stored them at a military base in Kamchatka, along with a number of cryogenically frozen Demogorgons. Following Hopper, Joyce, Murray, Dmitri and Yuri's escape from the base in March 1986, the Mind Flayer particles were accidentally let loose. Hopper, Joyce and Murray returned to the base intending to destroy the particles, hoping to aid their allies in Hawkins by inflicting damage to the Upside Down's hive mind. However, upon their return, they discovered that the escaped particles had possessed the bodies of the frozen Demogorgons, freeing them. They were forced to change their plans, aiming to kill the Demogorgons instead, as doing so would also destroy fragments of the Mind Flayer.