Goat format ruling
Gravekeeper's Watcher
Card effect (current official text)During either player's turn, when your opponent activates a Spell/Trap Card or monster effect that could make them discard when it resolves: You can send this card from your hand to the Graveyard; negate the activation, and if you do, destroy it.
Rulings
- You can activate the effect of Gravekeeper’s Watcher to negate Graceful Charity.
- You cannot activate Gravekeeper’s Watcher during the Damage Step (such as against a Morphing Jar that is flipped by an attack).
- You cannot activate Gravekeeper’s Watcher when a card is discarded as a cost, such as Magic Jammer, Kuriboh, Tribute to the Doomed, Thunder Dragon, or Type Zero Magic Crusher.
- You cannot activate Gravekeeper’s Watcher against your opponent's Delinquent Duo because it makes YOU discard, not your opponent.
- You cannot activate Gravekeeper's Watcher against the activation of your opponent's Mirage of Nightmare because it is possible that your opponent will not have to discard (for example, by having more than 4 cards in his hand).
- When Morphing Jar is Flip Summoned, you may chain the effect of Gravekeeper's Watcher, negating the Flip Effect of Morphing Jar and destroying it.
- Q: Which card effects can the effect of "Gravekeeper’s Watcher" be used against? Can it be used against "Mirage of Nightmare?" A: It works against card effects that would require the opponent to discard cards from their own hand, but not if the discard was a cost. "Graceful Charity" and "Card Destruction" come to mind. There really aren't many options. You cannot use "Gravekeeper's Watcher" against "Mirage of Nightmare."
- Q: Can "Gravekeeper’s Watcher" prevent the discarding of "Kuriboh" from a player’s hand to reduce damage to 0? A: Discarding "Kuriboh" is a cost of activating its effect. You cannot activate Gravekeeper's Watcher's effect when a card is discarded as a cost, such as "Magic Jammer," "Kuriboh," "Tribute to the Doomed," "Thunder Dragon," or "Type Zero Magic Crusher.
Rulings data courtesy of Format Library — formatlibrary.com. Spotted an inaccuracy? Flag it in the Goat forum — sourced change requests get reviewed.