Just as the Warriors of Light had to defend their world from becoming engulfed by darkness, the Warriors of the Dark were forced to defend their own world from being swallowed by the light many years ago.
At the end of Final Fantasy III, the party has to find their dark counterparts so that they can aid them in their battle against the Cloud of Darkness. Each Warrior of the Dark appears after defeating each of the four bosses guarding the Dark Crystals in the World of Darkness: Cerberus, Echidna, Ahriman, and the Two Headed Dragon.
After each battle, one of the Warriors of the Dark tells the Warriors of the Light important information for the upcoming battle with the Cloud of Darkness. Before the final battle, the Warriors of the Dark sacrifice themselves so that the Warriors of the Light can save their own world.
The Warrior of Darkness is seen briefly at the epilogue of Heavensward after King Thordan has fallen, being sent by Elidibus to stop the Warrior of Light to restore the balance of Light and Dark. He shares his appearance with the Midlander Hyur poster boy.
The Warriors of Darkness officially appear in Patch 3.1, defeating Ravana at the Smoldering Wastes. They then turn their attentions to the Scions and the Warrior of Light, but are interrupted by a familiar Hyur, and flee after revealing who they are and warning that they will meet again.
Following Nidhogg's final defeat at the Steps of Faith, the Warriors of Darkness retrieve his eyes from the chasm and present them to Elidibus, who later gifts it to the Ala Mhigan extremist the Griffin in hopes that he would cause another Calamity. They later appear at Xelphatol and reveal their origins and their reasoning for allying with the Ascians; they were originally Warriors of Light from another world, but in defeating the Ascians who sought to destroy their home, the aetheric balance was shifted heavily towards Light, causing a Flood of Light. They then allied with the Ascians in hopes that they could restore their world.
During their final confrontation with the Scions at the Bowl of Embers, Urianger, having played both sides in order to seek an audience with Minfilia Warde, now Hydaelyn's avatar, aids the Scions in defeating the Warriors. The Scions emerge victorious, and with the Warriors of Light and Darkness's Crystals of Light, both groups are brought to Hydaelyn's realm. Minfilia offers to take the Warriors of Darkness back to their world to stop the Flood. Upon their return, she and the Warriors of Darkness save their leader sacrifice themselves to contain the Light, leaving Norvrandt as the only landmass.
In Shadowbringers, the Warrior of Light, having been transported to the First to save it to prevent the Eighth Umbral Calamity, takes on the mantle of the Warrior of Darkness and aids the survivors in the fight against the sin eaters. During their journey there, they reunite with Ardbert, the leader of the Warriors of Darkness they encountered at the Source, who wandered Norvrandt for a century as a shade (time passes much faster in the First) following Minfilia and his friends' sacrifice. Ardbert, unable to interact with anyone or anything save the Warrior of Light, accompanies them and slowly regains his sense of hope.
With Hydaelyn's Blessing, the Warrior is seemingly able to absorb and contain the corrupted light aether from Norvrandt's Lightwardens. Upon defeating the final Lightwarden, they are unable to contain the corruption, slowly transforming into one themselves. During the Warrior of Light's final confrontation with the Ascian Emet-Selch, Ardbert appears and merges their souls together to stabilize the corruption, allowing the Warrior to weaponize the light to defeat the Ascian.
During the role quests, the Warrior of Light teams up with various Sin Eater hunters who hope to slay the Cardinal Virtues, powerful Sin Eaters made from the bodies of Ardbert's companions with Vauthry's corrupting power who are subconsciously driven by their past desires. The Warrior learns of Ardbert and his friends' origins and that they were accompanied by a sixth member named Cylva.
Upon slaying all four Cardinal Virtues, Cylva herself reveals that she set up the hunt for them, hoping that someone would put her friends to rest. She also reveals herself as the Shadowkeeper, who was instructed by the Ascians Loghrif and Mitron into bringing about a Rejoining, and her origins as another survivor of the Thirteenth's destruction. She begs the Warrior of Light to kill her to atone for her sins, though they convince her to live and carry the legacy of her friends.
At the beginning of their adventure, the Warriors of Darkness are chosen by the Crystal of Lux, and are blessed with its power. During their journey, they find five crystal shards which bless them with the Ranger, Dark Knight, Dancer, Ninja, and Magus Jobs.