Using Dispersal fails to “return opposing creature with the greatest cost” and only performs the discard effect.
Played against Gingerbrute in the Trick or Treat? event, which had a twilight prophet out most of the game.
The expectation is the Twilight Prophet would be returned to the opponent’s hand, or be destroyed if the hand was full. However, Dispersal failed to remove the Twilight Prophet from the board.