Halla was one of the Migrant Fleet's founding vessels, but was not originally of quarian manufacture. Then known as Sietor, she was an independently-owned turian merchant ship that happened to be visiting Rannoch for trade when the exodus began. Rather than fleeing, Sietor's captain elected to land the ship on Rannoch's surface - a risky move, given the ship's low speed and lack of manoeuvrability - and take on as many survivors as possible before relaunching and joining the general retreat.
Sietor remained with the nascent Migrant Fleet until the immediate danger was past, and some semblance of order could be formed among the fleet vessels. Given the nigh-impossibility of transferring Sietor's cargo of refugees to other vessels, already crowded with survivors of their own, the ship's captain and crew - in addition to being hailed as heroes - accepted what for the then-impoverished Fleet was a significant payment in exchange for ownership of Sietor herself. Though renamed Halla, the vessel's original name, and the names of her crew at the time of the exodus, remain on display on a commemorative plaque on the bridge.
Following Halla's purchase, as soon as more pressing needs were seen to, Fleet engineers undertook considerable modifications to make her systems compatible with the quarian technology in common use on the Fleet. Over the coming decades the ship was altered significantly, donating certain parts to other vessels while receiving others from decommissioned or damaged ships in return, resulting in an improvised hybrid turian/quarian design. Given the challenges inherent in such a design, priority was placed on reliability and long life; thus, while a hardy vessel, the Halla did not excel in any particular field, and was not suitable for specialised duties.
Despite her mundane capabilities, Halla's distinguished origin as a Fleet vessel has been upheld by a succession of well-respected captains, and generations of her crew developed a reputation for dependability and diligent service to the Fleet.
Halla served with the Civilian Fleet during the Battle of Rannoch. Due to her unusual design she was ill-suited to a number of the more advanced tactical systems installed in most civilian vessels, and was able to accept only the more unremarkable guns and defenses at the Fleet's disposal. She saw action, but was not advanced to the front line - her role was to be ready to fight a rearguard in the event of disastrous failure of the offensive. During a momentary incursion by geth vessels into the quarian lines she suffered damage and some casualties, but overall emerged relatively unscathed.
Given the more advanced quarian and geth ships available when the unified fleet was organised, Halla was not assigned offensive duties. She travelled to the Sol system as an armed supply vessel, ferrying munitions and replacement parts to damaged or depleted combat ships where possible in order to return them to the battle. Halla remained in the Sol system during the mass relay blackout, and subsequently returned to Rannoch.