Nin, the Pain Artist

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.

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

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.