2011-09-22 : If the ability resolves, but the damage is prevented or redirected, the controller of the targeted creature will draw X cards, regardless of how much damage was dealt or what the damage was ultimately dealt to.

2011-09-22 : If the creature is an illegal target when Nin’s ability tries to resolve, it won’t resolve and none of its effects will happen. No player will draw cards.

2011-09-22 : If the damage is lethal, the cards are drawn before the creature is destroyed (the next time state-based actions are checked).

2011-09-22 : The effect is not optional. The creature’s controller can’t choose to draw fewer than X cards.

2011-09-22 : You may choose a value for X that’s greater than the creature’s toughness. Its controller will draw that many cards.