“Stranger Things” season 2 memorably ends with the heroes (primarily Eleven) preventing the all-powerful Mind Flayer from making its way into our world from the Upside Down, only for that nasty bugger to find another entrance near the start of season 3. This was also when the show doubled down on its previous body horror, with the Mind Flayer possessing countless rats at an abandoned steel mill and imploding them to create a grotesque physical body — a creature that Pelletier and his crew dubbed “The Goop” — through which it could interact with (and, in time, even possess) people in Hawkins.
“Implode” is, in fact, the correct word here. “You’re not looking for guts and blood to splatter all over the place like a grenade effect,” Pelletier noted. “You want the rats to sort of flip inside out, instantly. It would splatter, but not separate into isolated chunks.” This presented challenges of its own, he explained:
“The problem was, as much as the Duffers loved gore, they said they would have a hard time getting it through Netflix if it was too descriptive — that is, once the rats start flipping inside out, we had to be careful not to make it too biological, where we can see the heart and the lungs and the eyeballs. It had to be something that looked nasty but on that thin line between spooky-cool and outright gory.”
To accomplish this, Pelletier and his team devised “a system using an animated rig rat asset and a recipe from the creature effect department. We could make it flip inside out and hide the fur instantly in a thick pile of nondescript guts and blood and mucus.” The end result perhaps isn’t quite as disturbing or visceral as ’80s monster movie practical effects at their nastiest (think the dog-Thing hybrid from John Carpenter’s “The Thing”), but it certainly gets the job done.