Operation Archon is the major showdown at the end of Final Fantasy XIV: A Realm Reborn It was the Eorzean Alliance's response to Gaius van Baelsar (the Black Wolf) issuing an ultimatum to the city-states and beast-tribes with his trump card: The Ultima Weapon. Operation Archon was drafted up by the Scions of the Seventh Dawn as a multi-stage plan to cripple the XIVth Legion and destroy the Ultima Weapon inside Gaius' stronghold.
In-game, Operation Archon composes the final segment of the V2.0 Main Scenario quests. The player is mainly involved in eliminating Rhitahtyn sas Arvina at Cape Westwind, and directly infiltrating Castrum Meridanum and The Praetorium. The remaining battles occur off-screen.
In order to cut off support from sea, the Aethernet Gate at Cape Westwind had to be captured while the Lominsan navy would encircle Castrum Marinum. Rhitahtyn sas Arvina was placed in command of the base, and was personally at the Aethernet gate. His death comprised step one of Operation Archon.
With Cape Westwind and Castrim Marinum gone, the next part was cutting off the phantom train supply lines to Castrum Oriens and Ala Mhigo. The Order of the Twin Adder laid siege to Castrum Centri.
The final step was for the Immortal Flames to march through Raubahn's Push and pin down Gaius in his stronghold. The barrier in Castrum Meridianum was the only obstacle left to confronting the enemy. A strike team entered the outer fortress to disable the shield generators, slaying Livia sas Junius in the process.
The airship Enterprise allowed Cid nan Garlond to infiltrate the Praetorium with a team of adventurers to destroy the Ultima Weapon. As they descended through the immense fortress, the Magitek Reaper previously used to enter Castrum Centri was discovered inside the lower levels and blasted through concrete walls and past Ultima's Arms.
Before they could reach the Ultima Weapon, Nero tol Scaeva was confronted outside the lift elevator. Though defeated, he made his escape while exalted about Ultima Weapon's activation. As they descended the lift, Gaius appeared and was battled on the elevator. Though a match for the adventurers, he walked into the room with the Ultima Weapon.
Getting inside, Gaius battled the party from inside the Ultima Weapon cockpit. Despite its immense power from having absorbed three primals, the light of the Mothercrystal gave the party an edge against the superweapon and gradually removed the primals. As Gaius vexed in frustration, the Ascian Lahabrea appeared and explained that the shield of light could only be removed by bringing "the hammer of darkness." He activated the Heart of Sabik inside the Ultima Weapon, and released the spell Ultima.
Though it destroyed much of the Praetorium, the party was shielded by Hydaelyn, who use the last of her strength to do so. Gaius attempted to defeat the adventurers once again, but the Ultima Weapon still fell before their might. Lahabrea sought to confront the party directly, as the Scions were interfering with his plans to resurrect of "the one true god." The Light of the Crystals banished him from Thancred's body. As the player escaped with an unconscious Thancred on the Magitek Reaper, Gaius accepted his defeat as the Praetorium was consumed in fire, though he survives its destruction and lives on as the Ascian-hunting mercenary Shadowhunter.
With the Ultima Weapon and Gaius van Baelsar defeated, the Eorzean Alliance celebrated on the shores of Lake Silvertear declaring the dawn of the Seventh Astral Era. But an ear-splitting roar dampened the celebrations, reminding them that the Primal troubles are far from over.