Whenever enchanted player casts a spell, put a spite counter on Curse of Vengeance.
When enchanted player loses the game, you gain X life and draw X cards, where X is the number of spite counters on Curse of Vengeance.
Curse of Vengeance’s first triggered ability resolves before the spell that caused it to trigger. The ability will resolve even if that spell is countered.
If you and the enchanted player both reach 0 or less life at the same time, you’ll lose the game before Curse of Vengeance’s second triggered ability gives you more life.