The Black Rose (黒薔薇, Kurobara?) is a Garlean alchemical weapon in Final Fantasy XIV. Being of arcane design, the Black Rose is a deadly gas that kills whoever breathes it by halting their aether to a standstill.
The Black Rose was developed in Gyr Abania during Gaius van Baelsar's time as Legatus of the XIVth Legion. Gaius abandoned all production of the gas condemning it as a weapon for extermination rather than conquest. The Ascian Elidibus, while possessing the body of crown prince Zenos yae Galvus, recommissioned its production when the Empire grew tired of the rebellions in its conquered territory following the liberation of Doma and Ala Mhigo.
Following the revelation of the gas's existence, coming across one of its testing sites, Gaius is joined by Alphinaud Leveilleur in destroying the plant manufacturing it. No longer a Legatus, Gaius informs the Scions of the Seventh Dawn and the Warrior of Light of the gas, relaying the information to the Eorzean Alliance. Though Alphinaud and Gaius destroy the main production facility, the Garleans construct several more to produce mass amounts of the Black Rose for the renewed war in Ala Mhigo.
After the Warrior of Light and most of the Scion leadership are transported the the First, Tataru and Krile recruit Estinien to continue the investigation on the Black Rose in their stead. While infiltrating a Garlean facility suspected of containing the weapon, he encounters Gaius. As their goals align, they agree to an alliance, and travel to Garlemald to investigate the matters further.
On the First (one of the reflections of the Source world), the Warrior of Light meets with Urianger Augurelt, who had also previously traveled there. From the time-traveling Crystal Exarch, Urianger learned of the future: the war between the Eorzean Alliance and the Empire turned in the Alliance's favor, and they began pushing the Empire further and further back. Faced with defeat, the Empire unleashed the Black Rose in desperation. The weapon proved much deadlier than even the Empire expected, decimating not only Eorzea, but spreading to the imperial provinces as well. With nearly all known nations on the Three Great Continents devastated by the Black Rose, the land descended into chaos—an Eighth Umbral Calamity. Among the casualties of the Calamity were the Warrior of Light and the other Scions.
Seeking to prevent this future from coming to pass, Urianger delved into a three-year study of aetherology during his stay in the First. He finds that the Source's understanding of active aether as "Astral" and static aether as "Umbral" is the reverse of what the people of the First believe. As the color white is produced by the absence of other colors, the First came to call static aether "Light," while the color black is produced by the absorption of all colors, so they call the active aether "Darkness." He and Thancred Waters muse that while the Source named their active and static aethers after their effect, the First named them after their generation, and that the First's interpretation may be the correct one.
With this new information, Urianger deduces that since the Black Rose operates by bringing its victims' aether to a standstill, it must have a strong affinity with static or "Light" aether. Having uncovered the process of Rejoining, Urianger explains that as the First reaches true stasis, the Flood of Light would begin leaking light aether to the Source. He concludes that in the future, the Empire's use of the Black Rose coincided with the First being completely consumed by the light and Rejoining with the Source. This unleashed a massive amount of light aether on the Source, resulting in Black Rose's unexpected potency and causing the Eighth Umbral Calamity.
Back on the Source, Gaius and Estinien arrive in Garlemald to find that the true Zenos yae Galvus has regained possession of his body and assassinated Emperor Varis. Zenos proclaims that he killed his father not out of any desire for the throne, but to prevent him from using the Black Rose to kill the Warrior of Light and denying him his hunt, leaving the fate of the Black Rose project unknown.
Black roses are symbols featured in fiction with many meanings. The symbolism in many works of art or fiction is usually to contrive feelings of mystery, danger, death, or some sort of darker emotion like sorrow or obsessive love.