Text (WL+errata): As an additional cost to cast ~this~, discard X cards. ; ~this~ deals X damage to each of X target creatures and/or players. [Oracle 2009/07/08]
Cannot be cast for a value of X more than the number of legal (different) targets. [Duelist Magazine #19, Page 34] You cannot target the same thing more than once. [Fifth Rulebook, Page 37]
