The Ossus Invasion took place circa 3628 BBY. This military campaign was executed by the re-established Sith Empire, initially under the command of Darth Malora and subsequently Darth Malgus. The Sith's primary goal involved the annihilation of a Jedi agricultural settlement that Master Gnost-Dural had founded following the Eternal Empire's conquest of the galaxy.
In 3636 BBY, the Eternal Empire, helmed by the former Sith Emperor Vitiate, launched an offensive against both the Sith Empire and the Galactic Republic. This aggressive action compelled numerous surviving members of the Jedi Order to seek refuge. By 3630 BBY, Jedi and Sith, under the leadership of the Eternal Alliance's Commander, initiated an uprising against the Eternal Empire, which eventually led to the demise of the former Sith Emperor.
During the aforementioned conflict, Master Gnost-Dural envisioned Ossus as a place where colonists could cultivate crops to enhance their self-sufficiency, while Jedi Knight Tau Idair advocated for dispatching teams to locate surviving Jedi. However, Gnost-Dural refrained from taking such measures to ensure the Jedi remained concealed from the Sith. Sometime after the conclusion of the war against Zakuul, the Sith Empire discovered that Jedi Master Gnost-Dural was alive and in hiding at the Jedi agricultural colony on Ossus. Consequently, the Empire dispatched an assault force to the planet under the command of Dark Council member Darth Malora, while Republic General Daeruun endeavored to reinforce the colony before the Sith could obliterate it. Given the Eternal Alliance's diminishing influence in the galaxy, they feared a conflict with both major factions and chose to support the Jedi and Republic, with the Alliance Commander arriving to aid in the defense of Ossus.
Following the events on Ossus, both the Republic and the Sith Empire became aware that Darth Malgus had survived his presumed death above Ilum. Darth Malora's position on the Dark Council became vacant after Malgus assumed control of her operation on the planet. Additional Republic and Imperial forces arrived at the planet, including numerous Valor-class cruisers, Thranta-class corvettes, and Harrower-class dreadnaughts, along with their accompanying fighters.
The choices made by the player in the Iokath expansion dictate whether Darth Acina or Darth Vowrawn appears as Empress/Emperor in the Sith storyline. Furthermore, the player's class determines which companions they reunite with during the Ossus storyline: the Sith Warrior class with Jaesa Willsaam, the Jedi Knight class with Doc, the Jedi Consular class with Nadia Grell, and the Sith Inquisitor class with Khem Val. Imperial classes also encounter Moff Pyron, a non-companion character from the Inquisitor storyline.
The conclusion of the storyline presents differing fates for Gnost-Dural based on faction allegiance. If the player aligns with the Empire, they can either capture Gnost-Dural as a prisoner to gain Light Side points or allow Malgus to kill him for Dark Side points. If the player sides with the Republic, Malgus inflicts severe injuries upon Gnost-Dural, leaving him in the library. Idair and the player subsequently discover a barely alive Dural and transport him to Doc for medical assistance.
Moreover, the storyline addresses the farming data that could potentially provide sustenance for either faction amidst the galactic resource scarcity. A loyalist player secures the data for their faction, whereas a saboteur player fabricates the data's destruction and delivers it to the opposing faction.