Spell Rupture
Instant
Counter target spell unless its controller pays 
, where X is the greatest power among creatures you control.
"Like water, mana will drain from an inferior vessel." —Jalan Tosk, Simic forcemage
Kev Walker
Gatecrash (GTC)
• #52Rulings
The value of X is the greatest power among creatures you control when Spell Rupture resolves. If you control no creatures at that time, X will be 0. The target spell’s controller can choose to pay {0} (simply by indicating they wish to do so). That player can also choose to not pay {0} and the spell will be countered.
(wotc • 2013-01-24)
Rulings
The value of X is the greatest power among creatures you control when Spell Rupture resolves. If you control no creatures at that time, X will be 0. The target spell’s controller can choose to pay {0} (simply by indicating they wish to do so). That player can also choose to not pay {0} and the spell will be countered.
(wotc • 2013-01-24)