Text (GP): 1/4. ; {X},{Tap}: Each creature with converted mana cost X can't attack or block this turn.
If you play the ability after you declare attackers, but before your opponent declares blockers, then it will not affect your attackers, but it will prevent your opponent from declaring blockers of the appropriate costs. [Guildpact FAQ 2005/12/22]
The ability affects the game rules for the turn, so it affects creatures already in play and even ones that enter play this turn after the ability resolves. [Guildpact FAQ 2005/12/22]
