The Contents of that Briefcase Henry Opens in Stranger Things?

For much of the fifth and final season of the popular Netflix show, Holly Wheeler and Max find themselves trapped within a psychic prison constructed from the recollections of the villain Henry. Their quest for an escape route in the pivotal sixth episode forces them to experience a defining event from Henry's childhood—an event that potentially carries the means to overcome him permanently.

Heads Up: This discussion details significant events for the second part of the final season.

The Discovery in the Cavern

After locating Max in the world called Camazotz, Holly discovers she is hiding out in a grotto that Henry strangely fears. An accidental discovery a telescope taken from Henry's old home helps Holly understand that the danger lies not within the cave. Matching her view to the cut-out, she and Max tumble into a deep mine shaft.

In this place, they observe a child version of Henry come across an injured and paranoid man. When Henry tries to help, the man fires a gun, hitting him in the hand. Fighting for his life, Henry strikes the man down with a rock. "Is this what corrupted him?" Holly wonders aloud. Essentially, yes, but the trauma wasn't just a defensive killing.

Similar to other plot points in volume 2, the significance of the encounter was first explored the theatrical production The First Shadow.

An Interdimensional Incident and Secret Science

*The First Shadow* opens with the legendary naval test, which transported a U.S. Navy ship into a realm later called the Abyss. The ship was attacked, and the only one to return was the ship's commander, who came back in a catatonic condition with an entirely new blood.

This man's son, the future Dr. Brenner, attempted to recreate this phenomenon in the Nevada desert. One of his scientists took experimental components to sell to the Soviets, who were eager to begin their parallel research. This spy in the cavern is the one who stole the materials.

So, What's in the Case?

Considering what is understood of Brenner's work, the briefcase almost certainly contained vials of the altered blood. At that moment, Henry is bleeding from his hand. He likely becomes contaminated upon contact with the blood, causing the development of his psychic abilities—abilities he would genetically impart to the children in Hawkins Lab.

Another possibility, the box could have held dark matter that eventually found its way to the secret facility, where it helps stabilize the mirror dimension. The prequel shows that Henry was transported in the Abyss, reappearing 12 hours later with a new blood type and personality. Throughout the play, he fights the influence of a sinister presence, similar to how the character Will was possessed by the entity in season 2.

Confronting Terror

Henry's enduring terror associated with the mine shows how utterly horrifying the event was for him. While he commands the army of monsters and attempts to pull the Abyss into Hawkins, a piece of the scared boy lingers. Vecna uses fear, and the series is building toward a showdown between the monster and Will, who has overcome his own deepest fears to become stronger.

This journey has effectively disarmed Vecna. Crucially, Will now knows what Vecna himself fears, and this may grant a decisive edge in the series finale ahead.