Single Strike Energy.

This card can only be attached to a Single Strike Pokémon. If this card is attached to anything other than a Single Strike Pokémon, discard this card. As long as this card is attached to a Pokémon, it provides Fighting and Darkness Energy but provides only 1 Energy at a time, and the attacks of the Pokémon this card is attached to do 20 more damage to your opponent's Active Pokémon (before applying Weakness and Resistance).