how did henry creel become vecna

Henry Creel becomes Vecna after Eleven blasts him through a portal into the Upside Down, where the dimension’s violent energy mutilates his body and he reshapes the realm with his psychic powers into the monstrous being later called Vecna.
Early life as Henry Creel
Henry grows up in the Creel family in 1950s Hawkins and begins to manifest powerful psychic abilities, including telekinesis and visions.
He becomes increasingly alienated, obsessed with predation and decay, and eventually uses his powers to torment his family, leading to the Creel House murders that are later blamed on his father.
From Henry to Subject 001
After the Creel incident, Dr. Brenner takes Henry to Hawkins Lab, strips him of his name, and designates him Subject 001, keeping him under tight control because of how dangerous his powers are.
Brenner implants a device in Henry’s neck to suppress his abilities, forcing him into a role as both test subject and, later, a kind of orderly who secretly manipulates the other children.
Manipulating Eleven and the Lab Massacre
Henry befriends Eleven, convincing her that Brenner is the real enemy and persuading her to remove the device that is limiting his powers.
Once freed, he unleashes his full strength, massacring almost everyone in Hawkins Lab, but when he tries to recruit Eleven to his vision, she resists him.
The banishment into the Upside Down
During their showdown, Eleven taps into a powerful positive memory, overpowering Henry and blasting him through a rift she tears open in reality.
He is hurled into the primitive Upside Down, where lightning and dimensional forces burn, tear, and distort his body, stripping away his human appearance.
Becoming Vecna
Rather than die, Henry uses his vast psychic abilities to dominate the creatures and hive-mind of the Upside Down, eventually molding its particles into the spider-like entity fans know as the Mind Flayer.
In this process he fully sheds his former self, taking on the name and role of Vecna—a controlling, parasitic predator who can invade minds, curse victims, and open gates between Hawkins and the Upside Down.
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