Stranger Things secretly concealed the first rematch between Eleven and Vecna. Stranger Things season 4 stripped away the show's secrets and revealed a villain who had been working in the shadows all along. Named Vecna by the Hawkins kids, Henry Creel was Eleven's oldest enemy, a being who possessed all her powers and potentially even more.
Unlike Eleven, Henry Creel's powers appear to be entirely natural. They seem to have developed around puberty, and he slaughtered most of his own family before he fell into the care of Dr. Brenner. He became the basis for all of Dr. Brenner's work; when the scientist realized his Number One could not be controlled, he began conducting experiments to create others who possessed Henry's abilities. Brenner's experiments were a success, but he made the mistake of keeping the — apparently depowered — Henry as an orderly at Hawkins Lab. Ultimately Henry was able to manipulate Eleven into freeing him, and he slaughtered the other Hawkins children before being cast into the Upside Down by Eleven. He has waited there for years, working to get back to Earth.
Stranger Things actually secretly concealed the first rematch between Vecna and Eleven. According to Stranger Things season 4, Vecna's mind now lies at the heart of the hive mind binding all life in the Upside Down together. It's not that the Mind Flayer is his servant; rather, Vecna is the Mind-Flayer. This retcon transforms a scene in Stranger Things season 3, where Eleven confronted the Mind Flayer's consciousness in a psychic conversation. Presumably, Vecna chose to conceal his true identity as he spoke through Billy, explaining some of the strange pronouns used during the conversation. He left Eleven terrified, perhaps in part because she sensed something familiar lurking behind Billy's eyes.
Was The Mind Flayer Controlling Billy Part Of Vecna's Plan?
Stranger Things season 4 retconned Vecna as the real villain all along. It's Vecna's consciousness that drives the Mind Flayer, binding all life in the Upside Down to his will, and even drawing upon them for energy. According to this retcon, Vecna was surprised when he realized Eleven was powerful enough to close the Gate in season 2 and chose to make her his primary target. He absorbed several Hawkins residents into the hive mind, transforming them into his thralls, but he swiftly realized Billy was of particular value. It wasn't just that Billy was the first; it was also that Billy seemed to be the one Vecna's enemies had identified. He knew they would reveal themselves if he drew them out.
It's important to remember Vecna hadn't seen Eleven through human eyes since 1979, meaning she'd grown up a lot. He had no way of recognizing her until she began to demonstrate her powers. It was only when Eleven used her powers in battle with Billy that Vecna knew for sure who his real enemy was. From that point on, Stranger Things season 3 became a strange game of cat-and-mouse as Eleven hunted Vecna, while Vecna, in turn, hunted her; Eleven made a mistake in opening her mind to him, giving him an opportunity to figure out where she was and target her with his creations. Fortunately, he had underestimated Eleven's friends and loved ones, who managed to shut down the Russian Gate. This particular defeat led directly into Stranger Things season 4, with Vecna deciding he clearly couldn't depend on Gates created by others; he had to make a way for himself.