- If a player attacks with Gengar & Mimikyu's "Horror House GX" attack, will the opponent be able to attach an energy card from their hand via Eternatus V's "Power Accelerator" attack or Shadow Rider Calyrex VMAX's "Underworld Door" Ability?
- No, you can't do anything that takes a card from your hand and puts it in the play area after your opponent uses Horror House GX. That includes Abilities, card effects, and attack effects.
Source:
TPCi Rules Team (2021-07-01)