What did Jezebel threaten to do to Elijah?

Answer: Kill him

Jezebel sent a message threatening to kill Elijah by the next day.

“So Jezebel sent a messenger to Elijah, saying, “May the gods deal with me, and ever so severely, if by this time tomorrow I have not made your life like the lives of those you killed!””— 1 Kings 19:2 (BSB)