They go back into your hand, so the best cards to sacrifice are ones with good ETB/LTB abilities, preferably cheap ones
Attack 1 : [2W] Aqua Return (120) Shuffle this Pokemon and all cards attached into your deck
We also now know that Illustration Rares and Special Illustration Rares, a card type that debuted in Crown Zenith , will return with this set, with ex cards being the mechanic we'll see depicted
Or, you can get creative and target your own Mirrorwing with pump spells or targeting cantrips like Expedite, rebounding to all your creatures
Just when you thought Charizard couldnt get any cooler, this cards Delta Species attribute makes it a Darkness-type Pokmon
We have an entire guide on how to build a Pokmon deck, but here's a quick summary