The Ultima Weapon is a creature from Final Fantasy XIV: A Realm Reborn. It is the final boss of the game.
It is a relic of ancient Allag, excavated from its resting place deep beneath Ala Mhigo. The Allagans used it to crush primal and foe alike...and now the Garleans mean to do the same!
The Ultima Weapon is a biotechnological, anti-primal war machine created by the Allagan civilization ages ago that was placed in stasis deep beneath what would later become Ala Mhigo. However, after the Seventh Umbral Era, the Ultima Weapon was found by the Garlean Empire and entrusted to Gaius Van Baelsar, who restored by having it absorb the primals Ifrit, Titan, and Garuda to assimilate their abilities. With this, he intended to use it to intimidate the city-states of Eorzea and the beast tribes.
Rejecting the terms of the ultimatum, the city-states instead reinstated the Alliance and launched Operation Archon to strike against Gaius's XIVth Legion and eliminate the Ultima Weapon itself at the Praetorium. Van Baelsar piloted the warmachine himself in the final battle, but despite his efforts the Light of the Mothercrystal shielded the party. As the Ultima Weapon took damage, the absorbed Primals were gradually released.
As Gaius became frustrated with the apparent strength of his opponents, Lahabrea appeared and explained the only way to break the shield of Light is to bring down the hammer of Darkness. He then revealed the core of the Ultima Weapon, the Heart of Sabik, held a power that lain dormant for eons: Ultima. Lahabrea activated Ultima, devastating the Praetorium and draining Hydaelyn of the means to shield the Adventurer. Despite being on more even terms, the party managed to destroy the Ultima Weapon before Ultima could be cast again. As it disintegrated, Gaius ejected from the failing war machine.
Following the defeat of the Ultima Weapon the Wandering Minstrel is encountered in Revenant's Toll and asks the Adventurer to recount their battle with the Ultima Weapon in order to make a song about the climax of the battle in The Prateorium. Through his song the Adventure finds themselves re-living their encounter with the Ultima Weapon, but can't help but notice that the Wandering Minstrel took liberties with the story and the battle is much fiercer than they originally remember.
The Ultima Weapon is fought during the second and third stages of the final battle, piloted by Gaius. In the first fight, it has access to several primal abilities, including their respective Astral Flows, but once enough damage is done, each primal is forced out of it. Ultima Weapon gains new abilities for the third battle, including the Ultima spell. If Ultima is successfully cast in battle, it's an instant loss.
Patch 2.1 launched with a Hard Mode variant of the battle with the Ultima Weapon. Emerging victorious from this battle will yield items stronger than Darklight equipment but not as strong as equipment found in The Binding Coil of Bahamut.
|"The Maker's Ruin"|
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The Ultima Weapon has 2 battle themes, one for each phase of battle. Both themes were composed by Masayoshi Soken.
The theme played during the first fight is called "The Maker's Ruin", the song contains a Latin chorus chanting.
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The second part of the fight is fought to the song "Ultima". The theme is used again in the battle with the superboss version of the Ultima Weapon in the sidequest "Minstrel's Ballad".
Similar to "The Maker's Ruin", this theme contains a Latin chorus chanting. The chanting in this song was randomly generated and as such there are no official lyrics. 
Ultima Weapon appears in Pictlogica Final Fantasy as a boss.
Ultima Weapon from Final Fantasy XIV has a Triple Triad card.
Ultima is Latin, and in many of today's Latin-based languages, for "The Last", in its feminine sense. However in the Spanish versions of many Final Fantasy games the spell name is "Artema", a closer transliteration of the original Japanese katakana.