Mind Flayer

"It was like this huge shadow in the sky. Only, it was alive. And it was coming for me."

- Will Byers

The Mind Flayer is a giant omnipotent creature from a dark, decaying version of Earth called "The Upside Down"

Biology
The Mind Flayer, from a distance, resembles a giant shadow of a five-legged spider-like creature with a long narrow head and each leg having an octopus-like movement. It can easily tower over a hundred feet and can cover more ground with its enormous legs. It is often seen surrounded by a red thunderstorm, though it often referred to as the "Blood storm" is unknown where it comes from. When seen up close, the Mind Flayer resembles a giant amorphous black cloud or fog and has fog like abilities, such as using its physiology to move its body through small entrances and pathways like that of a building.

Abilities
The Mind Flayer's abilities are somewhat vague, mainly because of how it is able to use them outside its home dimension. It can travel through rifts connecting Earth and its home dimension "The Upside Down". The Mind Flayer also has the ability to hijack people on Earth from The Upside Down without going through a rift. The Mind Flayer also has a bizarre hive-mind connection with the creatures and foliage of the Upside Down reminiscent of the Gravemind and The Flood from the Halo series. This hive-mind ability allows a psychic connection with the denizens of the Upside Down, meaning whatever knowledge learned by the Mind Flayer is transmitted to the other creatures and allows the Mind Flayer to send telepathic commands to the creatures. However, this hive-mind connection is so strong that if the creatures were ever injured, The Mind Flayer would feel the pain.

Weaknesses
Just like the other creatures of the Upside Down, the Mind Flayer is especially vulnerable to hot or warm temperatures (because of how the Upside Down is generally defined as being a very cold environment the creatures have gotten used to). Just as previously mentioned before, The Mind Flayer can't tolerate hot or warm temperatures, whether having hijacked somebody or not. Another prominent obstacle present for the creatures of the Upside Down are characters or entities who are psychically inclined or have telekinetic properties who can close the rifts connecting Earth and the Upside Down, and while it is shown that people with telekinesis can kill a creature from the Upside Down, the farthest extent shown so far for The Mind Flayer is just holding it back from going through a rift to Earth.

Lore
Unlike the other creatures from the Upside Down, who generally act more like animals killing based on instinct, the Mind Flayer is more sentient and has some sort of goal for it hijacking the character Will Buyers and sending its legion of monsters through the rifts into Earth. Some theories on the Mind Flayer's true intentions are to take over planet Earth and terraforming it into an environmentally sustainable for the Mind Flayer and the other creatures.