Combat Celebrant

Extra-combat effects usually arrive as mana sinks you tax yourself with every turn, in the manner of Aggravated Assault or Relentless Assault; this walks that effect onto the battlefield as a single 4/1 body and dares you to protect it long enough to fire it once. Exert is the mechanism: you mortgage the creature's next untap step to squeeze one burst out of its attack, and where most exert creatures spend that tax on a self-contained bonus (a counter, a damage spike), this one bends the mechanic into an enabler. The exert untaps all your other creatures and grants a second combat phase, so everything that just swung swings again while the Celebrant itself sits tapped until you find a way around the skipped untap. That untap is not restricted to attackers: mana dorks held back, untap-payoff creatures, anything you control comes upright, which is where the effect turns dangerous outside pure aggression. The 4/1 frame carries the whole balancing weight. It announces a glass cannon that wants to die in combat, and the exert tax makes the extra-combat trick a single-use detonation rather than a repeatable engine unless you can untap or blink it. That fragility is the point: doubling an army's worth of combat damage would be untouchable on a sturdier body, so stapling it to a creature that dies to one point of removal or a chump block keeps the explosive turn honest.