Axgard Braggart

Boast exists to reward the swing you've already committed to, and this Warrior is the plainest teaching model of the mechanic: attack, then pay to untap and grow. The untap clause is the part that pulls the effect out of ordinary anthem territory. Because Boast activates only during the turn the creature attacked, and this one straightens itself back up as part of the deal, it turns a 3/3 attacker into a blocker that also keeps its +1/+1 counter, punishing an opponent who was counting on a favorable trade or a clean crackback. That is a different combat math than a static pump: the counter is permanent, so each turn the body grows and the defensive posture holds, which makes racing it costly over several turns rather than one. The once-per-turn restriction and the requirement that it must have attacked are the two dials keeping the growth honest; you cannot bank activations or grow a creature that stayed home. As a design, it sits in the long line of aggressive white creatures that convert unspent late-game mana into board presence, but the untap wrinkle gives it a role most of that lineage lacks: an attacker that does not have to choose between offense and defense on the same turn.