The Battle of Onderon represented the Mandalorian Crusaders' final desperate attempt to achieve success during the concluding phases of the Great Sith War, fighting for the Brotherhood of the Sith.
Following the directives of Ulic Qel-Droma to redirect the majority of his Crusaders to the Onderon system, the fleet carriers under Mandalore the Indomitable established holding patterns in orbit between the planet and its neighboring moon, Dxun.
Emerging from behind the lunar body, the Mandalorian crusaders were initially observed by Queen Galia from the Royal Palace situated within the walled city center of Iziz. Upon sounding the alarm, King Oron Kira mobilized the Beast Riders from Onderon's wilderness to confront the impending assault. After dispatching a request for reinforcements to the Galactic Republic, Oron and the other riders engaged the Mandalorians in combat.
In the skies above Onderon, the Beast Riders and the Mandalorians riding their Basilisk war droids demonstrated comparable combat prowess. Provoked by the jeers of Oron Kira and his forces, Mandalore inspired his troops, who then fought with intense ferocity. Just as the momentum seemed to shift in favor of the Mandalorians, Republic reinforcements, consisting of elements from the Republic Swift Fleet under the command of Fleet Captain Vanicus, entered the fray, providing crucial firepower to the battle.
Trapped between the Republic forces and the Beast Riders, the Mandalorian forces suffered significant losses. Although Vanicus initially aimed to disable the enemy ships, this proved unachievable due to their deflector-shielding. The only option was victory or death. Ordering a retreat towards Dxun, Mandalore the Indomitable hoped to evade Republic pursuit within its dense jungles. Regrettably for Mandalore, during the retreat, a Republic frigate targeted his Basilisk war droid, resulting in a crash within the forests, far from the other Mandalorian soldiers. Upon emerging from the wreckage, Mandalore was immediately attacked by two large creatures native to the moon, and was subsequently consumed.
The remaining Mandalorians initiated a comprehensive search of the moon's jungles, seeking any trace of their leader, but their efforts proved fruitless. However, one shocktrooper stumbled upon the remains of Mandalore the Indomitable's mask. By placing the mask upon his own head, he assumed the mantle of the next Mandalore, Mandalore the Ultimate, whose legacy would have a lasting impact on the galaxy for many years.