Curiosity

Enchantment — Aura

Enchant creature
Whenever enchanted creature deals damage to an opponent, you may draw a card.

2011-09-22:
“You” refers to the controller of Curiosity, which may be different from the controller of the enchanted creature. “An opponent” refers to an opponent of Curiosity’s controller.
2011-09-22:
Any damage dealt by the enchanted creature to an opponent will cause Curiosity to trigger, not just combat damage.
2011-09-22:
Curiosity doesn’t trigger if the enchanted creature deals damage to a planeswalker controlled by an opponent.
2018-03-16:
You draw one card each time the enchanted creature deals damage to an opponent, no matter how much damage it deals or whether it was dealt in combat.
2018-03-16:
If this Aura enchants an opponent’s creature, you won’t draw a card when that creature damages you. The creature has to deal damage to one of your opponents for the ability to trigger.