The 13 scariest and most shocking moments on season 4 of 'Stranger Things'

Season four brings back Dr. Brenner (Matthew Modine), the man who raised and trained Eleven in the Hawkins Laboratory, in the form of a flashback during its opening sequence.

After what seems to be a routine morning doing the crossword and getting dressed, Brenner works with Ten, one of the children at the lab. During Ten's tests, however, something goes very awry. After Ten senses that something is wrong, the door to the room they're in gets blown off its hinges, knocking Brenner out.

When he awakes, he navigates through a bloodied hallway and into the Rainbow Room. There, he finds a blood-covered, but still alive, Eleven (Millie Bobby Brown), surrounded by the gruesome, contorted bodies of the other children.

It's a shocking display of bloodshed right from the top.

The first Upside Down action of the season happens in a Hawkins High bathroom in episode one. After throwing up — and denying Max's help — cheerleader Chrissy (Grace Van Dien), a newcomer in season four, hears her mother's voice asking if she's ready to try on a dress, saying that she's loosened the back.

When Chrissy doesn't reply, the pounding on the door intensifies and a pair of gruesome feet become visible below the stall door.

"Open the door, Chrissy, or I'm gonna gut you like the fat pig that you are," Chrissy's mother's voice says, distorting into a deeper tone.

Chrissy screams for the threat to go away as the stall door shakes and the dim overhead lights rapidly flicker.

After going to Hellfire Club leader Eddie's (Joseph Quinn) trailer home to buy some drugs, Chrissy hears the chime of a clock. When she goes to find Eddie, she sees a monstrous version of her mother working on her cheerleading uniform and becomes trapped in a hallucination of her own home.

At this point, Eddie's unable to get her attention, finding her trapped in a trance. The lights begin to flicker — a familiar sign that something is afoot in "Stranger Things." In the hallucination, Chrissy encounters a plate of rotting, spider-infested food, and eventually Vecna himself.

"It's time for your suffering to end," Vecna says. In the real world, Chrissy's body begins to levitate and unnaturally contort, her bones snapping and blood flowing from her eyes.

Chrissy's death kicks off the investigation into Vecna, the new threat from the Upside Down that's menacing Hawkins in season four.

In California, Eleven is bullied by her classmates after moving from Indiana, particularly Angela (Elodie Grace Orkin). When she runs into Angela and her friends while out with her longtime Hawkins friends Mike (Finn Wolfhard) and Will (Noah Schnapp) in episode two, they decide to humiliate Eleven by cornering her in the center of the roller rink, circling and taunting her, and splashing her with a milkshake.

Eleven originally hides after the incident from Angela but also Mike and Will. As seen in the first episode, without her powers, Eleven mostly hasn't retaliated against Angela's bullying.

Later, however, she emerges from a storage closet and confronts Angela and her friends, asking them to apologize. When they refuse, Eleven follows them, grabbing a skate from someone's hand and smashing it into Angela's face.

In the second episode, Mike's older sister Nancy (Natalia Dyer) and her newspaper coworker Fred (Logan Riley Bruner) nose around at Eddie and Max's trailer park. Fred, however, begins to experience visions similar to those that Chrissy had in episode one.

It's pretty clear that he's going to be Vecna's next victim, but the moment of his death is still terrifying as he attempts to escape Vecna and the visions of his car accident from the previous year.

After coming across the presumed body of his dead friend, once again crawling with spiders, he encounters Vecna, who kills him in the same bone-snapping way he killed Chrissy.

With Will's mother Joyce (Winona Ryder) and her partner in crime Murray Bauman (Brett Gelman) in Russia and Eleven with former Hawkins lab director Dr. Owens (Paul Reiser), Mike, Will, and Will's older brother Jonathan (Charlie Heaton) are alone in California.

After "friends" of Dr. Owens inform them of Eleven's situation, Agents Wallace (Kendrick Cross) and Harmon (Ira Amyx) confine the teenagers within the Byers' home. Eventually, they decide to escape, calling Jonathan's friend Argyle (Eduardo Franco) to deliver a pizza and serve as a getaway car in the process.

However, when someone knocks on the door, it's not Argyle. Rather, it's a group of men — presumably, government forces hostile to Eleven — with guns, who immediately shoot Agent Wallace and grievously wound Agent Harmon, who dies in the car.

When Vecna attempts to take Max (Sadie Sink), preying on her grief for her dead brother Billy, she's able to get a glimpse of Vecna's domain within the Upside Down. In it, the contorted, vine-encased bodies of Chrissy and Fred have been gruesomely preserved, as they were in their last moments.

After his failed escape attempt, Eleven's adoptive father and the former Hawkins police chief turned Russian prisoner, Hopper (David Harbour) tells traitor Russian guard Dmitri (Tom Wlaschiha) about his family, and his time serving in what was presumably the Vietnam War, in episode five.

Through flashbacks during the show's first season, Hopper previously revealed that his daughter Sara had died young of cancer. In season four, he tells Dmitri that he believes it was his fault.

Hopper says in the episode that he was part of the chemical corps during the Vietnam War, and would have to mix up Agent Orange, an herbicide that Hopper says he was told was "harmless."

In reality, the US Department of Veteran Affairs has since recognized conditions such as certain cancers and hypothyroidism as presumptive diseases related to exposure to Agent Orange. The Department has also recognized birth defects among the children of veterans who were exposed to Agent Orange.

According to the Associated Press, Vietnam says that up to 4 million of its citizens were exposed to Agent Orange, with up to 3 million, including the children of those exposed, becoming ill as a result.

Based on what Hopper told Dmitri, his daughter suffered what were presumably effects of his Agent Orange exposure and died.

Hawkins High basketball star Patrick (Myles Truitt) is Vecna's third victim, and the monster kills him while Patrick and teammate Jason (Mason Dye) are pursuing Eddie in a lake.

While we don't get to see Patrick's visions while he's killed by Vecna in episode five the way we did with Chrissy and Fred, his death in the lake is still pretty horrifying. Between the dark water and those hideous, bone-cracking visuals, it's not a pleasant scene to watch.

Steve (Joe Keery), Nancy's former boyfriend who became a friend to Dustin (Gaten Matarazzo), plays the hero at the end of episode six, volunteering to dive down into the lake to search for the gate to the Upside Down.

The dark water of the lake, where Patrick had recently died, is already freaky enough, but the fish bones Steve finds and the eerie red Upside Down glow makes it even more so.

It's easy to see where things are going when a tendril snakes out from the gate, following Steve towards the surface. But when it wraps itself around Steve's ankle and drags him down, leaving him at the mercy of the Upside Down's demo-bats, it's hard not to be scared for Hawkins' best babysitter.

Joyce and Murray arrive in Russia seemingly just in the nick of time to save Hopper. However, after they infiltrate the Russian prison where he's being held, they realize that he's going to have to fight the first antagonist of "Stranger Things": the demogorgon.

While it didn't feel like Hopper was going to meet his end at the hands of the familiar monster, it was freaky to see the demogorgon back and kicking. Watching it rip apart other prisoners with its unsettling, flower-like maw while Hopper and Dmitri attempted to escape was nerve-wracking.

Barb (Shannon Purser) was Nancy's friend from the first season of "Stranger Things," and one of the demogorgon's earliest victims.

At the time of Barb's death, Nancy was dating Steve and brushed off Barb during a party. Sitting outside by the pool, the blood from Barb's cut hand attracted the monster to her. It pulled her into the Upside Down, resulting in her death.

Vecna reminds Nancy of this moment as she remains trapped in his trance, showing her Barb's corpse and making her relive her killing.

In episode seven, Eleven finally remembers what happened during the massacre at Hawkins Lab. The character credited as the "friendly orderly" (Jamie Campbell Bower) — aka One, aka Vecna, aka Henry Creel — tells her about his past as a member of the Creel family, and how he ended up at the lab. Simultaneously, Vecna himself explains to Nancy how he became Vecna.

Vecna explains that after his family moved into the Creel house in Hawkins, he began to exercise his powers, killing small animals and later torturing his family. However, after his father was arrested for the deaths of his mother and sister, he himself fell into Dr. Brenner's custody and became a subject within the lab — One.

After allying with Eleven, he massacred the subjects of Hawkins Lab. However, Eleven chooses not to join him, and when he attempts to kill her, she opens the first portal to the Upside Down and banishes him within. There, he became Vecna, the monster that would eventually torture the people of Hawkins.

It's a shocking twist and one that brings together the season's different threads, from Eleven's history at the Hawkins Lab to the Vecna murders.