I'm going to assume you meant
Rescue Energy from Triumphant, and not
Recover Energy. Please note that as this card has yet to be officially released in English, we can only speculate on the rulings that will be made regarding it (or
Gengar Prime, for that matter).
As I understand the card text for Rescue Energy, it will allow you to return the KO'd Pokémon to your hand if that Pokémon is knocked out by
damage from an opponent's attack. Gengar Prime's first attack is inconsequential to the defending Pokémon, and its second attack places damage counters rather than dealing direct damage; therefore, I do not believe that Rescue Energy helps the defending Pokémon in this scenario.
If, on the other hand, you had Gengar Prime with the
LV. X attached and KO'd the Pokémon in question with Compound Pain, I believe that Rescue Energy would actually kick in and return the defending Pokémon to its owner's hand. Nothing in the card text I'm seeing for Rescue Energy limits its effectiveness to Pokémon destined for the discard pile (they just have to be knocked out from damage), so I'm basing my conclusion on that.
As always, comments from other Professors are welcome.
