Dungeons & Dragons is a fantasy tabletop role-playing game first published in 1974. It is a popular activity with Mike, Lucas, Will and Dustin, who play the 1983 Expert edition of the game in Mike's basement. They use elements from the game to make sense of the inexplicable situations they find themselves in.
The campaigns of Mike, Lucas, Dustin and Will
Mike, Lucas, Dustin and Will played Dungeons & Dragons from at least 1979 onwards, according to Mike. They played their "Elder Tree" campaign in 1979, for which Mike's sister Nancy dressed up as an elf.
They played a ten-hour campaign on November 6th 1983, from about 10am to 8pm. Mike, acting as Dungeon Master, summoned an army of troglodytes before continuing to summon the Demogorgon, the Prince of Demons. Will was urged by Lucas to "fireball" and attack it while Dustin advised him to cast a protection spell. Will attempted to fireball it, but he only rolled a seven when he needed a fourteen or higher. This meant Will's character was defeated by the Demogorgon. In an eerie coincidence, Will was abducted by a monster in real life shortly after the campaign ended.
Later, Eleven flipped the game board upside-down to explain that Will was trapped in an alternate dimension, which they later named the Upside Down. Dustin compared the Upside Down to the Vale of Shadows from the game: "a dimension that is a dark reflection, or echo, of our world. It is a place of decay and death, a plane out of phase, a place of monsters. It is right next to you and you do not even see it." She used the Demogorgon figurine to explain that the Monster was present within the Upside Down like Will was.
After Will's return, the boys played a campaign on Christmas Eve. In the story, the characters encountered a "lost knight", a "proud princess" and "weird flowers" in a "cave". Eventually, Mike summoned the Thessalhydra, but this time Will triumphed over the enemy, fireballing the beast with success. The boys complained that the campaign was "way too short" and that Mike had failed to tie up the story in a satisfying way.
Behind the scenes
The game is used as a self-referential plot device in the series. The first campaign played in "Chapter One" foreshadows the events of season one with the Demogorgon appearing and taking Will as the Monster they would name Demogorgon did in the following scenes.
In real life, the "Vale of Shadows" is not a official part of D&D canon - however, a chapter from the 2002, D&D-inspired video game "Icewind Dale" shares the name. The "Vale of Shadows" could also be an alternative name for the location known as Shadowfell.
The final campaign in "Chapter Eight" can be interpreted as a comment on the length and unresolved storylines of the first season; the boys complain that the campaign was too short, questioning "What about the lost knight? And the proud princess? And those weird flowers in the cave?" There are different interpretations among fans of what each element refers to; the "lost knight" is thought to refer to Hopper or Eleven while the "proud princess" refers to Eleven or Nancy. There is mostly agreement that the weird flowers in the cave refers to the Monster and/or the eggs in the Upside Down.
The Monster is nicknamed after the Demogorgon, which in Dungeons & Dragons mythology has two heads, each with their own mind that strives to kill the other, but they are unable to do so since they are connected to one another. This lends support to a popular theory among fans that Eleven and the Monster are in some way connected. Many fans also believe the Thessalhydra's appearance in the final campaign foreshadows events in season two. Since the Demogorgon's appearance in the first campaign foreshadowed the Monster and the role it would play in season one, it's possible the Thessalhydra could similarly hint towards other monsters from the Upside Down.