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)
Abilities » Underworld Door
Can I use Shadow Rider Calyrex VMAX's "Underworld Door" Ability if there are no cards left in my deck to draw from?
Yes, you can. Attaching Psychic Energy is the main effect of the Ability, which means it can be used even if you have 0 cards in your deck.
Source: Chilling Reign FAQ; TPCi Rules Team (2021-06-17)
Abilities » Underworld Door
If I use Shadow Rider Calyrex VMAX's "Underworld Door" Ability, do I have to draw 2 cards?
Yes, it is not optional (it does not say "you may"). You must draw 2 cards if you are able to; you can only draw a single card if it's the last card in your deck.
Source: Chilling Reign FAQ; TPCi Rules Team (2021-06-17)