Advanced 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.
Current rulings for this card are maintained by Konami.
Rulings data courtesy of Format Library — formatlibrary.com. Spotted an inaccuracy? Flag it in the Advanced forum — sourced change requests get reviewed.