Kulrath Mystic

A blue creature that rewards you for casting expensive spells is a strange animal: blue's payoff engines usually reward spell count (prowess, storm) or spell type (noncreature, instant/sorcery), not raw mana value. This one flips the axis. The 2/4 body wants to sit back, and it does that job unaided; a four-toughness wall in a color that leans on cheap interaction makes a fine early blocker on its own. What the trigger adds is a conditional gear-change: land a big spell and the wall becomes a vigilant 4/4 that keeps holding the ground it just won. The +2/+0 and vigilance arrive together for a reason. Without vigilance, swinging with the bonus would leave you tapped out and open the way any blue attacker is; with it, the attack costs you nothing on defense, which is exactly what a creature built to bridge a controlling deck into its top-end needs to do. The design lives entirely in the threshold. Set the bar at four and you exclude the cantrips and cheap counters that fill most of a blue turn, so the buff has to be paid for by committing to the top of the curve: a bomb, a heavy sweeper, a finisher. It is a body that scales with how greedy the deck around it is, and it does nothing at all for the deck that only ever wants to Bolt-and-counter its way to the finish line.