Vecna successfully stole Eleven's powers when controlling the Mind Flayer in Stranger Things' season 3 ending, which finally allowed him to emerge stronger than Eleven and enact his plan to take over Hawkins with the Upside Down.
Season 4 finally introduced the true mastermind of the Upside-Down's invasion, the being at the heart of the Mind-Flayer's hive mind. Vecna is a dark mirror of Eleven herself, sharing all her powers, and potentially others as well.
Her abilities are fully restored after she untapped her memories of One's assault on Hawkins lab. Having fully processed the trauma of accidentally freeing a monster who killed her 'brothers and sisters,' her psychic powers returned to their full strength.
The leading theory was that Eleven lost her powers due to overexertion; however, Dr. Brenner confirms in season 4 that Eleven's powers were lost because of the Mind Flayer's attack at Hopper's cabin and later during the Battle of Starcourt.
Brenner was able to make or alter the other children so that they would also have powers. Because of that, Vecna was inherently more powerful because he had his powers longer and knew the ins and outs of his abilities.
At his empire's height, Vecna was betrayed and destroyed by his most trusted lieutenant, a vampire called Kas the Bloody-Handed, using a magical sword that Vecna himself had crafted for him, now known as the Sword of Kas.
Following Eleven's resistance, Vecna sought after Eleven's 'power', which he identified as "a means to open [his] own doors" (gates).
However, after the big fight sequence between her and the Mind Flayer in the Starcourt Mall, she realized she could not use her powers. It was only during the epilogue section of season three, when we see her pack for Lenora Hills, that it becomes clear that her powers had gone for good.
In episode 7, Vecna is revealed to be Henry Creel (played as a youngster by Raphael Luce), the son of Victor Creel (Robert Englund) and his wife Virginia (Tyner Rushing) shown in the 1950s flashbacks.
In a deeply buried memory, El discovers that she was the one who sent Vecna to the other dimension after an epic battle between the two. That knowledge sparks her powers back up.
Remember also that we have evidence that powers can be inherited -- El inherited her powers from Terri Ives, for example. So it stands to reason that Henry inherited his powers from one of his parents.
There are many questions about Eleven, like whether Eleven created The Upside Down, and in the season 4 finale, she brings Max back to life. Eleven uses her telekinetic powers to get into Vecna's mind and battle him herself.
While intending to get his revenge on Eleven and Brenner, Vecna also goes after victims who have dealt with traumatizing experiences in the past (reminiscent of his childhood and his time at the Hawkins lab).
One/Vecna had his powers blocked and was working as an aide at Hawkins National Lab where he befriended Eleven. Not knowing that One was evil, Eleven restored his powers which he used to gruesomely massacre the entire lab. Eleven banished One/Vecna to the Upside Down where he's been attacking the Hawkins teens from.
What is the significance of 4 for Vecna? The new season shows that the four chimes of the grandfather clock symbolize the four victims Vecna needs to claim in order to enter our dimension and take over humanity.
Vecna's Hand
This is a reference to the D&D character Vecna, for whom this Vecna is named; the D&D Vecna has a monstrous hand infused with dark magic. The design of Vecna's hand is also a nod to Freddy Krueger, the Nightmare on Elm Street villain who had a huge influence on this season in general.
Next up, the viewers go on to theorise that One/Vecna is actually Eleven's dad. Yep, told you it was wild. The theory claims that Brenner wanted to create a child with stronger powers, so he waited until One was old enough to become a father.
Then there were the three poor teens of Season 4. Vecna's first big credited kill was Chrissy Cunningham (Grace Van Dien). He followed that murder with canonically terrible driver Fred Benson (Logan Riley Bruner) and basketball star Patrick McKinney (Myles Truitt).
In Netflix's Stranger Things, Eleven develops a nosebleed whenever she harnesses her telekinetic powers. From crumpling a coke can to committing murder — the more exertion her psychokinesis requires, the more likely we will see her nose bleed.
Andrew Rich was a character introduced in the Stranger Things novel Suspicious Minds. Andrew was the biological father of Eleven and the boyfriend of Terry Ives.
Breakup. After being pushed away by Mike, Eleven breaks up with him and spends some much-needed girl time with her new BFF Max. They later reconcile after Mike reveals that he only avoided Eleven because Hopper had threatened him and tells El he loves her which she reciprocates.
Yeah, Bower says Brown was crying real tears during that scene. “She was terrified and freaked out. And Caleb [Heymann], our DP, was very smart and saw that she was having this reaction in the beginning of rehearsal and decided to shoot her first. It's so pure and beautiful,” Bower said.
And while his usual modus operandi is to embody his victims' negative self-talk (another very real psychological phenomenon), this time he is telling Nancy his plans and then releasing her so she can report back to everybody—Eleven included—that the end is nigh, with the goal of discouraging and disempowering them.
"Vecna seems to be actually intrigued by Nancy being able to figure him out, to be able to get this close to the truth... I think this is more Vecna's narcissistic traits that pull him to Nancy, why he wants to give her this information."