Chittering Harvester

• 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.

• 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 creature they control, then each other opponent in turn order does the same, knowing the choices made before them. Then all the chosen creatures are sacrificed at the same time.

Rulings via Scryfall