Exquisite Blood

Enchantment

Whenever an opponent loses life, you gain that much life.

2012-05-01:
Damage dealt to an opponent usually causes that opponent to lose life. An opponent paying life also causes loss of life.
2012-05-01:
If you and an opponent both lose life simultaneously, and this causes your life total to become 0 or less, you’ll lose the game before Exquisite Blood’s triggered ability can resolve.