All of this is stapled onto a pretty good template: a very aggressive double strike-vigilance-haste combination that allows it to attack as well as being activated as soon as it hits the battlefield
It has a tapped ability that gives one mana of any color that a land an opponent controls could produce
Being able to take out the biggest thing each of your opponents have is a strong effect for four mana, and the Spell Mastery effect, which is rather easy to turn on, can severely punish stompy players at your table
For Blastoise, this card regularly goes for about $600
#5: Roothpath Purifier I havent found a home for Rootpath Purifier yet
Boseiju, Who Shelters All For Boseiju, Who Shelters All to be worth the 2-life tax it imposes, you need to be casting big instants or sorceries that win the game if they resolve