Sahagin

The trigger asks for a specific kind of deck: not just spells, but expensive ones. That four-mana threshold reshapes the payoff entirely. Most spell-count creatures reward volume, counting each Opt or Consider toward some incremental gain; this one ignores the cheap glue and cares only about the heavy end of the curve. Every four-plus-mana noncreature spell you cast turns a 1/3 wall into an unblockable threat that keeps growing, so the defensive body is the point rather than a liability: it survives to become the finisher for a controlling shell that was already casting big spells to win anyway. The evasion clause matters as much as the counter. Because "can't be blocked this turn" is granted at the moment of the cast, the creature can attack profitably before its size would suggest it should, and a turn where you resolve two large spells stacks two +1/+1 counters onto a body that is already getting through. What balances it is the deckbuilding tax measured in actual mana spent: you cannot cheat the threshold with rituals or cost reduction below four, and a deck full of two-mana interaction leaves this body inert at 1/3. It rewards a specific archetype (spell-heavy midrange or control willing to run genuinely expensive noncreature spells) and punishes the tempo decks that would otherwise want a cheap blue two-drop.