The Fifth Imperium was basically a continuation of the Fourth Imperium when in the year 1252 Emperor Avery Aella Alkhalikoi was assasinated, leading to the rise of House Corrino as the ruling Imperial family. The assembly of nobility known as the Moot were renamed the Landsraad, and the Corrino Emperors established a personal elite corps of shocktroopers known as the Sardaukar recruited from the ranks of Orks.
In Year 1304, the laws against psionics were repealed.
Over the next 2000 years after the rise of the Corrino Padishah Emperors, the Fifth Imperium would expand to conquer all of Charted Space as was familiar in the era of the earlier Third Imperium. Non-human races such as K'kree, Vargr, Hivers, and Aslan, and even certain human races like the Zhodani retained a degree of local autonomy under Corrino rule, and even limited representation in the Landsraad.
By the year 3500, the Fifth Imperium, thanks to a extremely powerful form of jump drive on Imperial starships that dwarfed even the old jump 6 drives, was rapidly expanding across the Milky Way Galaxy and its satellite galaxies. However, the Imperium was coming into increasing armed conflict with the robotic Cylons.