In the Eye of Chaos

World Enchantment

Whenever a player casts an instant spell, counter it unless that player pays , where X is its mana value.

2004-10-04:
This ability triggers when the spell is cast.
2008-10-01:
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.