Attacks » Horror House GX | Attacks » Power Accelerator | Abilities » Underworld Door
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)
Attacks » Horror House GX | Trainers » Viridian Forest
Does Gengar & Mimikyu GX's "Horror House GX" attack prevent me from using Viridian Forest?
No, it does not. Discarding a card from your hand is not the same as playing a card from your hand; the card goes to the discard pile, not "in play".
Source: TPCi Rules Team (2019-08-22)