An eyewitness at the party tells ET that excited guests started arriving around 7:30 p.m. to the bash that had a "fantastic atmosphere," complete with ice sculptures, a nacho bar, Frankenstein cookies, bins filled with ice cold beer, a photo booth and a DJ.
Paris Hilton led the costume charge, dressed as a sexy Viking. The 34-year-old socialite-turned-DJ flaunted her figure in a strapless leather bustier with a matching skirt.
Proving once again that they're the hottest couple ever, Channing Tatum, 35, and Jenna Dewan-Tatum, 34, put their own sexy take on matching Dr. Seuss costumes. The Magic Mike XXL actor rocked a Cat in the Hat onesie with a floppy hat and bowtie, while his wife looked hotter than ever in a strapless leather top, velvet black gloves and fishnet tights.
Apparently, Jenna's tiny hat gave Channing a case of the giggles.
We have to agree with Seuss himself, who once said, "Adults are just outdated children." Clearly, but how can you not love it?
Gwen Stefani was also in attendance as a sexy (and bloody) cowgirl that had a slight Dorothy from The Wizard of Oz vibe. The 46-year-old singer styled her dress with a braided pigtail wig, cowboy hat, bright yellow tights, and floral cowgirl boots.
Her makeup -- which consisted of fake freckles and blood on her face and chest -- was the highlight, as you can see from the close-up she posted to Instagram.
Meanwhile, Fergie and her husband, Josh Duhamel, kept it classy, paying tribute to Chanel fashion designer Karl Largerfeld and his cat, Choupette. The 40-year-old Black Eyed Peas singer wore an all-white ensemble consisting of a body-hugging dress, bob wig, cat ears and fur vest while the 42-year-old actor interpreted the fashion icon's signature look perfectly.
"Karrrrrrrlllll and choupette... ooh la la," Fergie captioned an Instagram pic from inside the event.
Another hot celebrity couple in attendance was Cindy Crawford, 49, and Casamigos co-owner Rande Gerber, 53. They stepped out in edgy leather biker gear, mimicking members of the famed Hells Angels motorcycle crew. An eyewitness tells ET that Cindy was one of the last partygoers to leave, heading out around 2 a.m.
But not all of the stars went for the sexy look at the party. Stefani's The Voice co-star Blake Shelton was barely recognizable in a dark wig and beard. The 39-year-old country singer completed his costume with a Mexican Serape poncho over a flannel shirt and a red-and-white trucker hat.
Despite rumors that Shelton and Stefani are dating, an eyewitness tells ET that the two didn't show any PDA at the party. "Blake and Gwen were barely even socializing together," the source said. "They were talking to several other people separately."
David Spade also went for a less come-hither get-up, dressing up as a pizza rat. The 51-year-old actor wore a zip-up onesie and mouse ears and held a fake pizza throughout the night.
If you can't get enough of celebrities in costume, watch the video below to see some of the sexiest Halloween ensembles from past years.