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)
If a player uses Venomoth's "Dizzying Wind" attack and the opponent tries to play a U-Turn Board during their following turn and flips tails, does U-Turn Board go back into their hand or stay in the discard pile?
If the opponent flips tails U-Turn Board is discarded and does not return to your hand.
Source: TPCi Rules Team (2021-07-01)
If a player uses Venomoth's "Dizzying Wind" attack and the opponent tries to play a Stadium during their following turn and flips tails, will they have used up their Stadium for the turn? And what happens to a Stadium already in play, does it get discarded or not?
If the opponent flips tails that card is discarded and doesn't count as being played, so they can still attempt to play another Stadium. The existing Stadium would remain in play.
Source: TPCi Rules Team (2014-12-04); TPCi Rules Team (2021-07-01)
If a player uses Venomoth's "Dizzying Wind" attack and the opponent tries to play a Supporter during their following turn and flips tails, will they have used up their Supporter for the turn?
If the opponent flips tails that card is discarded and doesn't count as being played, so they can still attempt to play another Supporter.
Source: TPCi Rules Team (2014-11-20); TPCi Rules Team (2021-07-01)
If I attacked with a Rapid Strike Pokemon last turn, then use Ninja Boy to swap it with Zeraora V, would the Zeraora V's "Cross Fist" attack do damage to a Benched Pokemon?
No, all effects carry over, so it would be considered the same Pokemon.
Source: TPCi Rules Team (2021-07-01)
Both players have 1 prize left. Player-A uses Galarian Slowking V's "Word of Ruin" attack. Player-B attacks Galarian Slowking V with enough damage to KO it with the same Pokemon that got hit by "Word of Ruin". Who wins? Or is it a tie?
The KO from Player-B's attack happens before the end of their turn, so the game is over before Word of Ruin's effect gets to happen. Player-B wins.
Source: Chilling Reign FAQ; TPCi Rules Team (2021-06-17)
When I use Galarian Slowking V's "Word of Ruin" attack, is my opponent's Pokemon still Knocked Out if they retreat it during their next turn?
Since it is an attack effect on the Defending Pokemon, the effect is removed if that Defending Pokemon is no longer Active.
Source: Chilling Reign FAQ; TPCi Rules Team (2021-06-17)
When using Banette's "Resolute Spite" attack, can you put more damage counters on Banette than it has remaining HP?
Yes, you can.
Source: Chilling Reign FAQ; TPCi Rules Team (2021-06-17)
If you use Spiritomb's "Ghostly Cries" attack, can you put more damage counters onto a Pokemon than it would take to Knock It out?
Yes, you can. You place all the damage counters for Ghostly Cries first, then resolve any Knock Outs from the attack.
Source: Chilling Reign FAQ; TPCi Rules Team (2021-06-17)
For Spiritomb's "Ghostly Cries" attack, do you put any Pokemon Knocked Out by the attack into the discard pile before or after shuffling all the Pokemon in their discard pile back into their deck?
After. Shuffle the discarded Pokemon into the deck first, then resolve any Knock Outs from the attack.
Source: Chilling Reign FAQ; TPCi Rules Team (2021-06-17)