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An ability that triggers when you cast a spell resolves before the spell that caused it to trigger. It resolves even if that spell is countered.

If you have more than one opponent, this ability can trigger once during each of those opponents' turns.

In a Two-Headed Giant game, this ability triggers no more than once during each opposing team's turn.

This ability triggers only on your very first spell during an opponent's turn, not the first spell after the card is on the battlefield. If you cast a spell before it's on the battlefield (including if you cast this card somehow during an opponent's turn), the ability won't trigger.