This is compounded further by a vision of the future that Nancy Wheeler experiences when she is captured by Vecna in Stranger Things season 4. After being allowed to go free, she tells Eddie, Dustin, Robin, Steve, Lucas, Max, and Erica that she saw her mom, Karen, her brother, Mike, and her sister, Holly, all suffering a seemingly gruesome fate. More than this, Vecna shows Nancy the fate of Hawkins and the world, making the dangers that will be unleashed in Stranger Things season 5 even more frightening.
When recounting what Vecna has shown her, Nancy specifically mentions a "giant creature with a gaping mouth," the implication of which can’t be underestimated. This creature may be linked to the Dungeons & Dragons campaign that Will, Mike, Dustin, and Lucas play in the Wheelers’ basement at the end of Stranger Things season 1. In that campaign, the monster they face is the Thessalhydra, and there are several good reasons to believe that this is the creature that Nancy sees in her Vecna vision.
In Dungeons & Dragons, the Thessalhydra is huge and stands as high as 20 feet and is between 30 and 36 feet in length. More pertinently, its defining feature is a huge gaping mouth surrounded by eight heads. In this way, the Thessalhydra fits Nancy’s description of the creature that she sees in her vision in Stranger Things season 4.
Despite this superficial similarity, the psychological attachments Vecna has with Nancy and Will offer a larger hint that the monster in Stranger Things season 5 will be the Thessalhydra. Will’s connection to the Mindflayer and, by extension, Vecna is foregrounded for much of Stranger Things' first four seasons. In this way, Will's painting of Mike and the rest of their party fighting a creature with multiple heads can be seen as having an even greater significance. As a result, Will and Nancy’s connections to Vecna may mean that they are both seeing different details relating to the same element of his plan. Although Will tells Mike that the painting is Eleven’s idea, there is reason to doubt this since the painting clearly holds some personal importance to Will. More than this, at the beginning of Stranger Things season 4, Eleven’s letter to Mike describes Will’s painting as being a secret that he won’t show or share with anyone.
What is certain is that if the creature in Nancy’s vision in Stranger Things season 4 is the Thessalhydra, it will be a fittingly formidable adversary. Nevertheless, the Dungeons & Dragon link to Stranger Things season 1 and Will’s painting both offer hope that the monster can be overcome. This is because in the campaign Will defeats the Thessalhydra by throwing fireballs at it, and this is also reflected in Will’s painting, in which he is shown throwing blue fireballs at the multi-headed creature. Of course, Stranger Things season 5 being the last season of Netflix’s TV phenomenon has huge significance to this as potentially anything can happen before the show ends. As such, the Duffer Brothers using a story detail and monster mentioned in Stranger Things season 1 seems entirely plausible and in line with the events of season 4.