Hunt for the Thessalhydra is a short adventure published by Wizards of the Coast as a tie in to the "Stranger Things" franchise and part of the Stranger Things Dungeons & Dragons Starter Set. The adventure features characters, locations, and monsters inspired by that series. The adventure is suitable for characters of level 3-5.

The Dysmal Swamp is a coastal wetland that stretches over 100 square miles. Devoid of any urbanization, the ecosystem enjoyed its dank sanctuary. A community of bullywugs called the swamp home for generations, hunting and maintaining the balance of the land. The land also appealed to the Belange Company, a wicked fey-run enterprise making cheap magic items, who needed a secluded location far from both thieves and oversight. When the Belange Company established their magical workshop in the swamp, the bullywugs attempted to drive them off. However, the Belange Company’s supply of magical items provided enough firepower to protect their endeavor, and many bullywugs fell in the initial conflicts. The Belange Company began crafting scores of magical items to be sold across the region, using a cheap alchemical process that produced a nasty liquid byproduct glowing with chaotic magical energies. Rather than devise a new process, the Director decided to discharge the waste into the swamp. As production increased, so did the pollution, and the sludge began to warp the wetland. The region’s typically reclusive bungisngis population became aggressive. It is only a matter of time before these mutated swamp giants lay waste to bullywugs.