Cavern Whisperer

• WOTC Official Reminder Textofficial rules
  Mutate — If you cast this spell for its mutate cost, put it over or under target non-Human creature you own. They mutate into the creature on top plus all abilities from under it.
• WOTC Official Reminder Textofficial rules
  Menace — This creature can't be blocked except by two or more creatures.

• WWizards of the Coast2020-04-17
  Official Ruling
  As the triggered ability resolves, first the next opponent in turn order (or, if it’s an opponent’s turn, the opponent whose turn it is) chooses a card in hand without revealing it, then each other opponent in turn order does the same. Then all the chosen cards are discarded at the same time.

Rulings via Scryfall