Nin, the Pain Artist

• WWizards of the Coast2011-09-22
  Official Ruling
  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.
• WWizards of the Coast2011-09-22
  Official Ruling
  If the damage is lethal, the cards are drawn before the creature is destroyed (the next time state-based actions are checked).
• WWizards of the Coast2011-09-22
  Official Ruling
  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.
• WWizards of the Coast2011-09-22
  Official Ruling
  The effect is not optional. The creature's controller can't choose to draw fewer than X cards.
• WWizards of the Coast2011-09-22
  Official Ruling
  You may choose a value for X that's greater than the creature's toughness. Its controller will draw that many cards.

Rulings via Scryfall