It turns out Matt Duffer, co-creator and co-showrunner of Stranger Things, was playing Baldur’s Gate 3 when he and his brother, Ross Duffer, were working out how the final battle would go down, and Larian’s much-loved party-based role-playing game influenced how it would play out.

Stranger Things Season 5 ended with the group defeating Mind Flayer and Vecna. They also successfully got rid of the Upside Down, but with Eleven's sacrifice.

Stranger Things season 4 volume 2 explains how Henry/Vecna is connected to the Mind Flayer and what happened when he first entered the Upside Down.

Stranger Things Season 5’s biggest twist, that the Mind Flayer was the real villain all along, was pretty well signposted. The end of Season 4 had apparently turned the entire story upside down, with Vecna claiming he was the show’s real villain and that he’d created the Mind Flayer.