Lords of waterdeep "Assasinate Rivals" warfare quest card question.

The Warfare card "Assasinate Rivals," found in the Skullport expansion, has as one of its rewards "each opponent returns ?(random colour cube) from his or her Tavern to the supply." Who decides what type of cube is returned? The person who completes the quest, or the opponents who are losing the pieces? Thanks.

edit: Ruling seems to go to the victims of the quest. Thanks for your responses!