Merfolk in legacy is a powerful deck that relies on being able to cast creatures while countering spells. It is a deck that uses aether vial to "cast" creatures while holding up counter magic.
In standard there is no force of will or aether vial, however there is phyrexian mana. This is an attempt to mimic the ability of legacy merfolk to cast creatures while controlling the game.
Card roles in comparison to actual merfolk:
Augur of bolas is a silvergil adept
master of the pearl trident is a lord
phantasmal image is in the deck to copy master of the pearl trident, but it also has utility in copying either snapcaster or augur of bolas.
snapcaster mage is just good for any deck with this many spells
tarland the sky summoner is kind of like another lord, except instead of making your guys bigger he just makes guys. He is what really makes this deck playable in my opinion.
mental misstep is free countermagic, however it is so specific that I wouldn't run more than one
mutagenic growth is also free counter magic against red spells and some black spells.
vapor snag is an excellent tempo card no standard aggro control deck is complete without.
gutshot mimics free counter magic by being free removal for many of the prominent creatures in the format.
gitaxian probe draws cards, allows you to see their cards, and makes free bodies for tarland.
