Following their initial success with the First Sphere Expansion and their alliance with the formidable Kroot, the Fire Caste armies of the Tau Empire were swollen by billions of additional warriors to aid the fight for the Greater Good, albeit in a more mercenary fashion than was considered ideal. With engineering centres in every Sept working diligently, the Earth Caste provided innumerable innovations, but the invention demanded by the Ethereals -- faster propulsion-technology to drive starcraft -- eluded them. At last, the quantum leap came from Fal'shia Sept, where they finalised development of the ZFR Horizon Accelerator Engine. An ingenious design, this powerful new mechanism allowed Tau voidships to attain near-light speed. It was this device that would usher in the next phase of pr