The Mind Flayer (Stranger Things)

The Mind Flayer is gigantic spider-like malevolent creature and the true main antagonist in Stranger Things, that was first introduced in Season Two. It is an intelligent gargantuan entity which can exert control over other life forms, including an army of brainwashed "Demodogs".

History
His full history so far remains a mystery and it's not known how he first came into existence. All that's known is that after an interdimensional gateway was opened at Hawkins labs on November 1983, the Upside Down soon began spreading to our realm, allowing passage to the Mind Flayer, who infected and took control of the mind of Will Byers.