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Heidi Klum, a master of Halloween, teased her much-anticipated costume to her nearly two million followers on Instagram on Oct.2 ... but we're not quite sure what it is.

The 42-year-old model, who is well known for her over-the-top Halloween costumes and parties, gave us a sneak peek of what's to come at the end of the month.

"It's almost time for my favorite holiday! Costume prep time with the team at @prorenfx Can't wait for Halloween! 🎃" she captioned a photo of herself covered in a plaster molding with a seemingly-horned headpiece.

A short time later, Heidi shared more behind-the-scenes photos and videos of her face and body being slathered in a goopy substance.

"So yucky but so worth it in the end 🎃 #heidihalloween#heidiklumhalloween" she said with a photo of her face covered in goop.

"Slime me up @prorenfx 🎃 #heidihalloween #heidiklumhalloween," the German supermodel captioned a video with a shout-out to Prosthetic Renaissance, who specializes in make-up effects.

It seems as though the most special part of the holiday for Heidi is the suspense.

"I always like to surprise. I don't like to tell people what I'm gonna do because people have a certain expectation of what it's gonna be like. I like to just show up and not tell anyone what it's gonna be," she recently told US Weekly.

Over the past few years, Heidi has transformed into a psychedelic butterfly, an old woman, a gorilla, Cleopatra and more outrageous getups.