Patagia Viper

Flying
When Patagia Viper enters, create two 1/1 green and blue Snake creature tokens.
When Patagia Viper enters, sacrifice it unless {U} was spent to cast it.

The List

Commander 2015

Flying
When Patagia Viper enters the battlefield, put two 1/1 green and blue Snake creature tokens onto the battlefield.
When Patagia Viper enters the battlefield, sacrifice it unless {U} was spent to cast it.

Dissension

Flying
When Patagia Viper comes into play, put two 1/1 green and blue Snake creature tokens into play.
When Patagia Viper comes into play, sacrifice it unless {U} was spent to play it.

If this enters in a way other than announcing it as a spell, then the appropriate mana can't have been paid, and you'll have to sacrifice it.