Most Stranger Things fans probably won't get this reference, so here's a quick history lesson: Back in the 1980s, one of ABC's most popular sitcoms was a show called Perfect Strangers.
It starred Mark Linn-Baker as Larry, an uptight midwesterner who just moved to Chicago, and Bronson Pinchot as Balki, his unknown cousin from a small Mediterranean island. Balki moves in with Larry, and comedy hijinks ensue, mostly stemming from Balki's ignorance of American culture and Larry's neuroses.
This sketch, made for Jimmy Kimmel's late-night talk show, brings back the original Larry and Balki for Perfect Stranger Things. But the cousins have a new roommate: Dem O'Gorgon -- everyone's favorite TV monster. The three of them disagree on interior design (like the Christmas light alphabet wall), use each other's toothbrush (it is covered with green slime -- of course it was the demogorgon), and when the demogorgon gets mad, he banishes Larry and Balki to the Upside Down.
A good crossover also has cameos. That's right -- Mike, Will, Lucas, and Dustin all show up in a very special Halloween episode! Yes, that's right -- Stranger Things actors Finn Wolfhard, Noah Schnapp, Caleb McLaughlin, and Gaten Matarazzo all appear in this sketch.