Answer:
Their language (the Kamanit language)
Explanation:
The Kamanit came to earth and made promises to humans. They promised to make their lives better with various forms of technology - no more war, famine or energy problems. The Kamanit delegate went as far as inviting humans to their planet. Their technological interventions were tested and they brought results which made humans happy.
When the Kamanit delegate was done speaking, he left a book behind - a book written in their language. Humans decoded the title and saw that it translated to - To serve man. This made humans trust the Kamanit because the title of the book suggested that they wanted to look out for them.
But when the whole book was translated, it was discovered that it was a cook book. And that the Kamanits planned to use humans as food.
But it was already too late because thousands of humans already moved to their planet.
