Whenever a modified creature you control attacks, double the number of each kind of counter on it. Then for each nontoken permanent attached to it, create a token that's a copy of that permanent attached to that creature.
Arna is a counters-and-attachments payoff: suit up creatures with Auras, Equipment, and counters, then attack to double every counter and copy each nontoken permanent attached to the attacker. This snowballs board presence and stats explosively, turning one well-equipped creature into a game-ending threat while Arna herself flies, gains life, and dodges removal. You generally win through one or two overwhelming modified attackers backed by a steady stream of free Aura/Equipment copies.
Doubling counters every combat creates exponential power growth that quickly outpaces the table
Copying nontoken Auras and Equipment gives free value engines and can spread buffs across creatures
Flying, lifelink, and Ward—discard a card make Arna a resilient, evasive attacker in her own right
Esper colors offer the best counter-doubling, proliferate, and protection support in the format
Heavily reliant on a single modified creature, so targeted or sweeper removal sets you back hard
Vulnerable to combat tricks, blockers, and fogs since the payoff only triggers on attack
Setup-intensive: needs counters or attachments before Arna's ability matters
Three-color mana base and equip/aura costs can be demanding on resources
Copies generate extra protection and proliferate triggers while doubling its +1/+1 counter output.
Add more proliferate and counter-doublers (Hardened Scales, Branching Evolution, Inexorable Tide) to amplify Arna's trigger, and lean on cheap, high-impact Equipment and Auras that benefit from being copied. Improve protection with Swiftfoot Boots, Lightning Greaves, and the Mythos/Sword cycle to keep your attacker alive, and tighten the mana base with fast lands and ramp so you can deploy Arna and suit up on the same turn.
Legendary Creature — Human Knight
