Play Skull Fossil as if it were a Colorless Basic Pokémon. (Skull Fossil counts as a Trainer card as well, but if Skull Fossil is Knocked Out, this counts as a Knocked Out Pokémon.) Skull Fossil can't be affected by any Special Conditions and can't retreat. At any time during your turn before your attack, you may discard Skull Fossil from play. (This doesn't count as a Knocked Out Pokémon.)
During your opponent's turn, if Skull Fossil would be Knocked Out by damage from an opponent's attack, flip a coin until you get tails. For each heads, put 1 damage counter on the Attacking Pokémon.