The symptoms of a black widow spider are similar to those of Will's during his time as the Mind Flayer's host, where his mind was effectively poisoned by the spider's poison. The Mind Flayer tricks the soldiers into marching into their death in the scene where Will is trapped in the hospital in the second season of the show.
Vecna was able to bring life to the ultimate creature he's been dreaming about by using the Mind Flayer as a model for a black widow spider. The scariest creature to come from the Upside Down in season 3 was the Hawkins civilians, who were turned into the Flayed. The Mind Flayer's first victim was Will, who had been made mentally and physically weaker by his long stint in the Upside Down, which began the connection between the two creatures. Vecna's Mind Flayer monster is heavily influenced by the spiders he grew so close to in his youth, and he sees its formation as a way of becoming what he's always been destined to be. Vecna shaped the dust of the Mind Flayer in the Upside Down into a spider.
The show confirmed a popular theory that it was Vecna's master plan all along, and he was behind all of the attacks on Hawkins. Its habitat is surrounded by vines, which when touched, alert the Mind Flayer to someone's presence, like a spider's web. He gave form to the Mind Flayer in the shape of a giant spider. Black widow spiders are named due to their habit of consuming their mates, and the Mind Flayer is also named due to its habit of consuming mates. Will Byers was the host of the Mind Flayer in the second season of Stranger Things.
The Mind Flayer is being attacked by the gang in Hopper's cabin in season 3. It was shown to be a formidable enemy, much scarier than the Demogorgon from the season before, due to its surprising intelligence. Vecna has been trying to get to Eleven for years using a host of monsters, and has been acting as a sort of ruler of the Upside Down. The Mind Flayer was almost inescapable, and even calculated. The monster took the civilians and used them to build its own form. He describes them as ''misunderstood'' and ''solitary creatures'' to which he feels he can relate. Vecna explains that the black widow spider feeds on the weak.
Why did Vecna make the Mind Flayer from shapeless dust into something spider-like? In a similar way to how a black widow spider's bite can weaken its victim, it "bites" Eleven, causing her powers to temporarily weaken. The black widow spider only bites when it's trapped or threatened. Vecna likes the spider's ability to make order in chaos, and dislikes humans for being pests in their natural world. He admires the black widow spider because he found a nest of them in the Creel house.
As a young boy, Henry Creel had a fascination with spiders.