Vecna

Vecna is the main antagonist from the fourth season of Stranger Things. He is portrayed by Jamie Campbell Bower as an adult and by Raphael Luce as a child. 
A fearsome sentient creature aligned with the Upside Down, Vecna had humble origins as the human Henry Creel. After moving to a new home in Hawkins in March 1959, the young Henry discovered he had remarkable psychokinetic abilities, and could alter the perception of others. Motivated by a nihilistic and misanthropic philosophy, Henry used his powers to psychologically torture his family, eventually killing his mother and sister, and then framing his father for these crimes, which drove him to madness. 
These events would draw the interest of Dr. Martin Brenner, who took him to Hawkins National Laboratory to be studied; Henry was designated 001, becoming the laboratory's first test subject. Brenner implanted Soteria into Henry's neck, a miniature device that suppressed his powers and tracked his every movement; in Henry's own words, he had become a 'prisoner'. In time, Henry became one of several orderlies who assisted Brenner and attended to the lab. Brenner would go on to replicate Henry's powers in a number of child test subjects, who also had their movements restricted to the lab's confines. 
In 1979, Henry tricked test subject Eleven into removing the Soteria chip with her powers. In a fit of rage, Henry used his restored powers to brutally murder the other test subjects, and attempted to do the same to Eleven. However, Eleven stood her ground and ultimately overpowered him, sending him through a gate to the Upside Down; there, he gradually transformed into the being known as Vecna. He became an explorer of the realm, discovering an exotic, storm-like mass of particles, and used his powers to reconstitute it into a giant, spider-like entity: a being that would come to be known as “the Mind Flayer”.
In 1983, Eleven accidentally opened a second, non-temporary gate at Hawkins Lab, and Vecna, acting through the Mind Flayer, attempted to eradicate humanity, both in 1984 and 1985. In both instances, Eleven and her allies ultimately thwarted Vecna’s plans.
In 1986, Vecna resurfaced, taking a more direct approach in his attempts to destroy humanity. He began murdering a host of Hawkins residents, tormenting his victims with his curse powerand using their deaths to open a series of new gates. Dustin, Nancy, Max and others began investigating the murders, nicknaming the murderer 'Vecna' after a character from Dungeons & Dragons. The group’s efforts to defeat Vecna culminated in a dual assault; Eleven used psychic projection to confront Vecna within his own mind, while Nancy, Steve and Robintravelled to the Upside Down version of the Creel House to attack his real body. Despite heavily wounding him, the group could not stop Vecna from furthering his plans to annihilate both Hawkins and the wider world.