When you play Volcanic Eruption - Cost: XUUU Card Type: Sorcery Rules Text (Oracle): Destroy X target Mountains. Volcanic Eruption deals damage to each creature and each player equal to the number of Mountains destroyed this way. Ruling: 10/4/2004 Can be used with X equal to zero. This is useful if no Mountains are in play. Doesn't it have to have X legal target mountains? So, if you then use Spectral Shift to alter Mountains to Swamps - you can't use it to now target swamps, can you? If there were mountains in play and you shifted it, could you then change the targets to Swamps? Or, am I missing something?