The Mind Stealer

The Mind Stealer is a magical shrunken head with the body of a doll, and it has the power to steal minds. It appeared in the first book in the Goosebumps HorrorLand series, Revenge of the Living Dummy.

Revenge of the Living Dummy
Britney Crosby finds the mind stealer, then it disappears later in the story, when she discovers Slappy to be alive, she finds it, and uses it to steal Slappy's mind, causing him to drop dead. Slappy's last words in the book were, "I'll be back Britney, I'm a BAAAAD BOY!"

Trivia

 * Slappy came back at the end either because the Mind Stealer didn't work, or because its powers wore off.
 * The Mind Stealer should not be confused with the Shrunken Head from How I Got My Shrunken Head, although their looks are very similar.