A significant conflict occurred on the planet of Ossus. This clash involved the Jedi Order against a group of dark side followers called the Legions of Lettow, and it took place during the First Great Schism around 24,500 BBY. The Jedi successfully defended against the Lettow's assault, and then pursued them into the Core Worlds. A tale suggests that the location of this battle on Ossus eventually became the site for the Eye of Ashlanae.
Around 24,500 BBY, disagreements emerged within the Jedi Order of Force-users. These disagreements involved a dissenting group of Jedi known as the Legions of Lettow, who advocated for the use of the dark side of the Force.
Ultimately, a direct confrontation—later known as the First Great Schism—between the two factions became unavoidable. The Legions of Lettow, commanded by General Xendor and his partner, Arden Lyn, opted to bring the battle to the Outer Rim Territories planet Ossus, the Jedi Order's isolated base, to prevent the Galactic Republic worlds from becoming embroiled in a potentially bloody war. However, the Jedi successfully fended off the Lettow's attack.
Following this, the Jedi pursued the Legionnaires into the Republic-controlled Core Worlds, leading to conflicts on Chandrila, Brentaal IV, Coruscant, and Metellos. Lyn was eventually placed in a state of stasis that persisted until 4 BBY. In that year, she was awakened by agents working for the Galactic Empire, and she mentioned the battle that occurred on Ossus during a debriefing, parts of which were recorded by Grand Inquisitor Laddinare Torbin.
A legend, which had developed by the time of the Galactic Civil War, suggests that the site of the battle on Ossus gained a sacred status among the Jedi Order. It is said that in the planet's Eocho Mountains, the Jedi who survived the First Great Schism each carved a stone block the size of a bantha—an animal from Tatooine that averages 2.5 meters in height—and, over several weeks, used telekinesis to move them to the battle site.
Once there, the Jedi refined and fitted the boulders, constructing the fifty meter–tall ziggurat-shaped Eye of Ashlanae as a memorial to their fallen comrades. It was also believed that a simple stone platform, located at the center of a chamber deep beneath the structure, marked the exact spot where the final blow of the First Great Schism was struck—although the conflict had actually concluded with the Jedi's victory over the Lettow on the Core planet of the same name. The High Council of the Jedi Order used the chamber for deliberations, and the stone platform was also considered to be a powerful vergence in the Force.
The battle on Ossus was initially referenced in the 2009 reference book The Essential Atlas, which was written by Daniel Wallace and Jason Fry.