Whitney, The Troll-Mage

+4: All creatures target opponent controls transform into 1/1 Blue Frogs. Creatures lose their abilities. At the end of the turn, destroy them.
-5: Target opponent gains an emblem with "At the beginning of your upkeep, mill half of your library, rounded up".
-15: Players play a Magic subgame, using their libraries as their decks. Each player who doesn’t win the subgame loses half their life, rounded up.