Demogorgon

The Demogorgon is the nickname of the otherwise unnamed monster and one of the two main antagonists (along with Martin Brenner) in the first season of the Netflix Original Series Stranger Things. It is a creature hailing from a parallel dimension reminiscent to a dark reflection of the world (the 'Upside Down'). It has the ability to create doors to other dimensions, and feed on inhabitants from there. It was sent by the Mind Flayer to kidnap Will Byers.

It was portrayed by Mark Steger.

History
Nothing is known about the origins of the Demogorgon or its home, but it was discovered by an enhanced child named "Eleven," whose abilities were originally utilized by Dr. Brenner for espionage. During her encounter with the Demogorgon, Eleven touches it, and the creature to use her contact with it to build a "Gate" to the reality she knew, and thus releasing it into her world, where it hunts animals and people. During this time, it takes a child, Will Byers, to his dimension to feed on him, but Will escapes from it and hides from it within the Upside Down. As Will's friends and family look for him, Eleven secretly joins the search and helps them get clues about the whereabouts of Will. Meanwhile, Dr. Martin Brenner attempts to issue several cover-ups to prevent knowledge about the Demogorgon's threat.

The Demogorgon is eventually discovered during the course of the search, and while some characters focused on finding the truth behind Brenner's cover-ups, the rest try to find and eradicate the creature. The Demogorgon eventually encounters Nancy Wheeler, the elder sister of Will's friend Michael, and tries to kill and eat her. Nancy escapes from the Upside Down and, along with Will's brother Jonathan, develops theories on the creature's instincts. They later use this knowledge to lure it to them, while they stocked up on dangerous weapons and traps.

Meanwhile, Eleven and her new allies find a way to locate Will's presence in the Upside Down, and trick Brenner into letting them through the Gate to find him. The Demogorgon is engaged and severely injured by Nancy, Steve, and Jonathan, and the former escapes to the Upside Down to avoid destruction. It then returns to the real world, where it encounters and kills dozens of Dr. Brenner's agents, injures him, and attacks Will's friends Michael, Dustin, and Lucas.

But is engaged by a weakened Eleven, who was low on energy by this time. Eleven uses her powers to restrain the Demogorgon and releases a strong energy that tears the monster to pieces and causes her to vanish.

Legacy
Will Byers, having been in contact with the Demogorgon, has symptoms of coughing up small creatures similar to it, implying that it had offspring through Will. Its crimes had also inspired many changes in the characters, both positive and negative.

Abilities
The Demogorgon can create and use dimension portals when in contact with any life form foreign to its dimension. It is also physically strong and dangerous, more so than most predators. It is also intimidating in appearance, which can cause adversaries and victims to panic.

Trivia

 * It was listed on IMDb as simply "The Monster".
 * It shares a name with one of the Demon Lords in Dungeons and Dragons.
 * Due to no one knowing its true name, it is referred to as the Demogorgon by Mike, Lucas, Dustin, and Eleven.
 * Its true motive and intentions for its kidnapped victims were never revealed, however, it was most likely just a predatory creature looking for food.
 * It is incredibly tough, able to withstand massive amounts of bullets. Even so, it's not invincible, as fire greatly wounded it.
 * Episode six, "The Bathtub", is the only episode of season one that it does not appear in.
 * The Duffer Brothers said in an interview that "Obviously something happened to Eleven when she destroyed and killed that monster," effectively confirming its status as deceased.