Vecna serves as the final boss in season 4 of Stranger Things and after revealing his origins in volume 1, volume 2 includes his plans for the future and a bit more.
Henry Creel was a troubled boy with abilities throughout his childhood and once Dr Brenner got ahold of him there was no turning back. But what exactly happened that led to his current visage and connection to the Mind Flayer?
The entity from another realm
The Mind Flayer was the ‘big bad’ terrorizing the people of Hawkins in seasons 2 and 3 of Stranger Things. It formed a mental connection with Will ever since he was kidnapped and held in the Upside Down.
The Mind Flayer was responsible for sending the Demogorgon, the Demodogs and for possessing the people of Hawkins, referred to as the Flayed. It behaved as a hive mind and this fact helped the others combat it.
Another key point was that the Mind Flayer’s psychic connection only lasted as long as there existed an open gate to the Upside Down. Eleven and the others slowly closed down each gate, severing the Mind Flayer’s connection.
When the Mind Flayer possessed Billy, it revealed its plan to Eleven of coming into their world and ending everyone, but at that point, it was simply considered a generic villainous threat and nothing else.
The predator pulling the strings
When Vecna has Eleven subdued and gets ready to kill Max, Eleven mentions Dr Brenner’s death and tries to convince him that it was Dr Brenner who was responsible for how he turned out because of his abuse.
Vecna tells her that she’s the real reason he is that way. When she attacked him in 1979, he thought he was dead but he was actually transported to another realm.
After exploring for a while, he came across an entity, a hive mind within which he saw great possibilities. So he used his powers to form a connection with this entity and shaped it into the form of a spider, just like the Black Widows he kept as a child.
That was how the Mind Flayer got its physical form and bonded with Vecna.
Eleven then realizes all the attacks from the Upside Down have always been the doing of Vecna as he sought her power. Every time he tried to maintain a connection through a gate, she closed it down.
He then tells her that he’s got the upper hand now, and once he kills Max, he will take over their world and recreate it in his own image. However, he doesn’t succeed in his plan thanks to the efforts of Eleven and the others.
In the aftermath, Will says that ever since he’s returned to Hawkins his connection to the Upside Down has strengthened and that Vecna is still alive, even if he may be gravely hurt. This was another indication that Vecna and the Mind Flayer are linked.