Kanamit

The Kanamits are nine foot tall sapient humanoids which claim to originate on a planet far beyond the galaxy. They communicate by telepathy and have mastery over advanced technology, including highly efficient energy sources and the creation of force fields.

The aliens come to Earth claiming to bring peace and prosperity, sharing their technology willfully and carrying around a book which cryptographers find out to be titled "To Serve Man" in their language, seemingly confirming the Kanamits' intentions of benefiting mankind. However, at the same time hundreds of Humans have been boarding the aliens' vessels to visit their distant homeworld, the main text of the book is successfully translated and proved to be a cookbook.

Trivia

 * In the original short story by Damon Knight, the Kanamit were rather different from their portrayal in The Twilight Zone, as rather than being tall, bulbous-headed humanoids in robes, the Kanamit were short, hairy creatures resembling anthropomorphic boars.