mtg modern masters 2015 card list Edition MM2 | 249 cards (English) Modern Masters 2015 Financial Set

If there are additional costs to cast a spell, or if the cost to cast a spell is increased by an effect (such as the one created by Thalia, Guardian of Thraben's ability), apply those increases before applying cost reductions

Hammerhead Tyrant

This allows for the hydras you cast to scale in size alongside the game as it progresses and you gain access to more mana

Also, she gives all of your other permanents indestructible too

Effects that double card draw or prevent discarding can break the mechanic wide open

It also costs three mana if you dont sacrifice a legendary creature, but again were going for flexibility