Warning! SPOILERS for Stranger Things season 4, volumes 1 & 2
In Stranger Things, Vecna fashioned the Mind Flayer from shapeless dust into something spider-like - why? The horrifying creature, the Mind Flayer, was introduced in Stranger Things season 2, when it took young Will Byers as its host. It was shown to be a formidable enemy, something vastly scarier than the Demogorgon from the season before, largely due to its surprising intelligence. Where Demogorgon's are much like animals, albeit still terrifying, the Mind Flayer was almost omnipresent, and even calculated. In season 3, it managed to possess a good amount of Hawkins civilians, turning them into the Flayed, and was the scariest creature to come from the Upside Down at that point.
In the Stranger Things 4 season finale, the show confirmed a popular theory that it was actually Vecna's master plan all along, and he was behind all of the attacks on Hawkins. Vecna has been acting as a sort of ruler of the Upside Down, and has been trying to get to Eleven for years using a host of monsters. This includes sending the Demogorgon, but he also gave form to the Mind Flayer, in the peculiar shape of a giant spider.
Henry Creel/Vecna/Number One had a fascination with spiders since being a young boy. He forms a sort of kinship with them, describing them as ''misunderstood'' and ''solitary creatures,'' to which he feels he can relate. He admires the black widow spider in particular as he finds a nest of them in Stranger Things' Creel house. Vecna envies the spiders' ability to make order in such chaos, and dislikes humans for being ''pests'' in their natural world. Naturally, when he came upon the dust of the Mind Flayer in the Upside Down, Vecna shaped it into the thing he's long admired: spiders.
The Greater Meaning Of The Mind Flayer Being A Spider
The Mind Flayer doesn't just look like the eight-legged creature, it actually operates similarly to a black widow spider, showing that Vecna seized the opportunity to bring life to the ultimate creature he's long dreamed about. Even its habitat is surrounded by vines, which, when touched, alert the Mind Flayer to someone's presence, exactly like a spider's web. More specifically, Vecna explains that the black widow spider immobilizes their prey, and ''feeds on the weak.'' The Mind Flayer's first victim in Stranger Things was Will, who had been made mentally and physically weaker by his long stint in the Upside Down, which begins the connection between the two creatures. The poison of a black widow spider is reported to cause muscle spasms, difficulty breathing, as well as sweating and chills, which is also incredibly similar to Will's symptoms throughout his time as the Mind Flayer's host, where his mind was effectively poisoned. The black widow spider, reportedly, only tends to bite when it's threatened or trapped. This mirrors the Stranger Things season 2 scene where Will is "trapped" in the hospital as they try to understand his condition, and the Mind Flayer tricks the soldiers into marching into their death, symbolically resembling the deadly bite.
The Mind Flayer "bites" again in season 3 in Hopper's cabin, where it is being ruthlessly attacked by the gang, and therefore threatened. It "bites" Eleven, causing her powers to temporarily weaken in a similar way to how a black widow spider's bite can weaken its victim. Most famously, black widow spiders are named thus due to their habit of consuming their mates, said to be for protein, and the Mind Flayer also consumes in an almost parallel manner. The monster took the civilians of Hawkins, broke them down, and used them to build its own form. No doubt, Vecna's Mind Flayer monster in Stranger Things 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.