If your creature is not a legal target when your spell resolves then it fizzles. A spell that specifies any number of targets must have at least one of those targets to resolve.
Quote from: Kaleo42 on November 14, 2013, 12:39:04 PM
If your creature is not a legal target when your spell resolves then it fizzles. A spell that specifies any number of targets must have at least one of those targets to resolve.
Kalei is correct
608.2b (http://imtgapp.com/forum/index.php?action=imtg;area=rule;number=608.2b): The spell or ability is countered if all its targets, for every instance of the word "target," are now illegal.
Example: Sorin's Thirst is a black instant that reads, "Sorin's Thirst deals 2 damage to target creature and you gain 2 life." If the creature isn't a legal target during the resolution of Sorin's Thirst (say, if the creature has gained protection from black or left the battlefield), then Sorin's Thirst is countered. Its controller doesn't gain any life.
Example: Plague Spores reads, "Destroy target nonblack creature and target land. They can't be regenerated." Suppose the same animated land is chosen both as the nonblack creature and as the land, and the color of the creature land is changed to black before Plague Spores resolves. Plagues Spores isn't countered because the black creature land is still a legal target for the "target land" part of the spell. The "destroy target nonblack creature" part of the spell won't affect that permanent, but the "destroy target land" part of the spell will still destroy it. It can't be regenerated.
A lot of the text for this rule has been removed for brevity, but the examples are left in tact.