Keening Stone
Artifact
Oracle Text
, 
: Target player mills X cards, where X is the number of cards in that player's graveyard.
Rules for this card
-
To mill a number of cards, a player puts that many cards from the top of their library into their graveyard. See rule 701.17.
Rulings (1)
-
Wizards of the CoastThe value of X is determined as the ability starts to resolve. If the targeted player has five cards in their graveyard at that time, for example, this ability will put five more cards into that graveyard from the top of that player's library.