Rottenmouth Viper Aetherdrift choose one that hasn't been

choose one that hasn't been chosen this turn —

or Nightmare costs {1} less to cast

and Rats you control get +1/+1 until end of turn

Each creature you control assigns combat damage equal to its toughness rather than its power

When Sarkhan enters

Rottenmouth Viper Aetherdrift choose one that hasn't beenAs an additional cost to cast this spell, you may sacrifice any number of nonland permanents. This spell costs {1} less to cast for each permanent sacrificed this way. Whenever this creature enters or attacks, put a blight counter on it. Then for each blight counter on it, each opponent loses 4 life unless that player sacrifices a nonland permanent of their choice or discards a card.