Culling the Weak | Magic: The Gathering
Formats Culling the Weak is Legal in
Official Oracle Text for Culling the Weak
Add to your mana pool.
—Stronghold architect, journal
Official Rulings for Culling the Weak
4/15/2013 : You must sacrifice exactly one creature to cast this spell; you cannot cast it without sacrificing a creature, and you cannot sacrifice additional creatures.
4/15/2013 : Players can only respond once this spell has been cast and all its costs have been paid. No one can try to destroy the creature you sacrificed to prevent you from casting this spell.