Play Mysterious Fossil as if it were a Basic Pokemon. While in play, Mysterious Fossil counts as a Colorless Pokemon (as well as a Trainer card). Mysterious Fossil has no attacks, can't retreat, and can't be Asleep, Confused, Paralyzed, or Poisoned. If Mysterious Fossil is Knocked Out, it doesn't count as a Knocked Out Pokemon. (Discard it anyway.) At any time during your turn before your attack, you may discard Mysterious Fossil from play.
