Whenever a player casts a spell, counter it unless that player pays.
10/4/2004 This is not an additional cost, it is a triggered ability.
10/1/2008 This has the supertype world. When a world permanent enters the battlefield, any world permanents that were already on the battlefield are put into their owners’ graveyards. This is a state-based action called the “world rule.” The new world permanent stays on the battlefield. If two world permanents enter the battlefield at the same time, they’re both put into their owners’ graveyards.
10/1/2009 The ability triggers whenever any player (including you) casts a spell of any type.