The Stranger Things series finale was released on Netflix on Dec. 31, and fans got to see the Hawkins gang one last time as they faced the ultimate battle against Henry Creel/Vecna. But while the last episode and final season as a whole answered some burning questions, it also left many mysteries unresolved. One such mystery concerns Henry's cave flashback/memory.
Spoilers are ahead from Stranger Things 5.
In the fifth and final season of Stranger Things, we discover that Max's consciousness is trapped in Henry's mindspace. Meanwhile, her physical body is lying in a bed at Hawkins Memorial Hospital. While venturing through Henry's memories, Max and Holly come across a traumatic one from when he was a little boy. We only get to see part of the memory at first, but it's fully revealed in the series finale. The memory goes like this.
One day, a young Henry stumbles across a cave and enters it only to find an injured man with a briefcase. Frazzled, the mysterious man points his gun at Henry. He thinks someone sent Henry to get him. What happens next is a physical altercation that ends with Henry bashing the man's head in with a rock before taking the briefcase and opening it. What lies inside is a glowing black rock whose particles enter Henry's bloodstream and ultimately give him his powers. According to Netflix, the particles were Mind Flayer particles.
Basically, this scene shows us how Henry got his powers. However, it doesn't go into detail about who the mysterious man in the cave was or where the rock even came from. Now that the show has wrapped up, the Duffer Brothers aren’t quite providing all the answers. But they have revealed that the upcoming Stranger Things spinoff will explore these "lingering questions."
In a new interview with The Hollywood Reporter, the Duffer Brothers shared that the man in the cave was a scientist. However, they don't reveal his full backstory. This, along with the origins of the black rock, will be addressed in the spinoff.
"I’ve been pinned down and forced to blow this already, so I might as well tell you. There’s lingering questions about the rock and where the rock came from [in Henry’s story in the finale] and the scientist and all of that. Because we had said that there is something in the finale that is going to connect to the spinoff. The spinoff is not about rocks or mining the rocks, but I would say that’s the loose end that’s not that’s not tied up that will be tied up. But ultimately, and we have said this before, but I guess to reiterate, it is an entirely new mythology. This spinoff does connect and will answer some of the lingering questions. It’s not specifically about the Mind Flayer or the Upside Down, but hopefully it provides some answers to that at least those lingering questions related to Henry’s memory." - Matt Duffer
Although the series doesn't explain who the scientist was, the prequel Broadway play, Stranger Things: The First Shadow, reveals that he was a rogue scientist who worked for Dr. Brenner. After stealing dangerous experimental technology, he hides out in the cave. Henry then comes across the technology in the cave, and it accidentally activates and transports him and the scientist to Dimension X/the Abyss.
It's unclear whether the scientist will have the same backstory as in the play in the spinoff, but that would make the most sense. However, the black rock isn't included in the Broadway play at all. That means the spinoff will provide new information about the rock and its origins.