"What if he never left? What if we locked him out here with us?" a terrified Mike asks Eleven.
"No matter what happens, we have to stop him," he continues.
But the creepy voiceover comes with the warning: "We're going to end you. We're going to end your friends. Then, we are going to end everyone."
The shock scenes which take place in a fairground show Stranger Things 3 is packed with the '80s fashion, music and nostalgia that fans love.
The Starcourt Mall, a new addition to Hawkins, makes an appearance in the trailer, as the backdrop for a giant battle with the monster.
And a host of familiar faces are back for the third season, including Winona Ryder, David Harbour, Gaten Matarazzo and Caleb McLaughlin.
Stranger Things 3 drops July 4 on Netflix.