The Nova Terra Interregnum, also known as the "Time of Twin Empires," was a troubled period in Imperial history when the Imperium of Man fractured for a time into warring factions for over 900 standard years during the 33rd Millennium. During this period, the rebellious Ur-Council of Nova Terra dismissed the authority of the High Lords of Terra and claimed separate rule over the Imperium's Segmentum Pacificus, founding their own pocket empire, known as the Nova Terra Regency or the Imperitor Nova. After nearly nine centuries of constant warfare, an uneasy stalemate was called, and the Imperium of Man reluctantly recognised the new empire. Today the Nova Terra Regency stands as a beacon of secular thought and reason in a galaxy otherwise bathed in ignorance and superstition. They possess considerable military power, second only to the rising Tau Empire in the east and the massive Imperium of Man itself.
"There is no darkness to be found between the void of stars, nor the deepest pits of the earth that equals the darkness of Mankind's deeds."
― Excerpt from the suppressed Neo-Terran Credos
Beginning in 075.M33, the Ur-Council of Nova Terra declared independence from the rule of the High Lords of Terra, and declared the whole Segementum Pacificus independent of Imperial rule. They had grown disenfranchised with the burgeoning Imperial state religion, in the form of the religious organisation known as the Ecclesiarchy, that zealously promoted the worship of the Emperor of Mankind as the one, true God of Humanity. As the power of the Ecclesiarchy waxed strong, the Ur-Council of Nova Terra foresaw the dangers of having a religious organisation possess so much power and influence over the citizens of the Imperium. They felt that the Imperium had lost its way and no longer followed the tenants of the Emperor's enlightened philosophy of the Imperial Truth. The Imperium of Man no longer upheld the core values of reason, science and secular progress. Instead, they had fallen back on the old ideas of religion, superstition and faith. To compound the secession of the Segmentum Pacifucs, two Space Marine Legions, the feral Space Wolves and the stoic Iron Hands, joined the rebellious Ur-Council of Nova Terra. The two Primarchs were later followed by a number of Legion Masters as well, who also joined the Secessionist's cause.
In response, the High Lords of Terra call for a new Founding to take place. The Third Founding, known also as the 'Numroi Founding', which took place in 098.M33. Utilising accelerated gene-culturing techniques not used since the First Great Crusade, three Space Marine Legions were created in record time. These few Legions were created to be massive, hosting hundreds of thousands of Legionaries within their ranks. But the shortcuts taken in their creation would have unforeseen flaws. These particular Legions came to be characterised as being battle-hardened, and particularly bloody-handed and unforgiving. The primary purpose of their creation was to help bring the Nova Terra Interregnum to a brutal and swift conclusion. The conflict that ensued over the next eight centuries, threatened to tear apart all that the Imperium of Man had rebuilt following the Age of Darkness of the Horus Heresy. Not since those dark days, had Legion been pitted against Legion.
Several of the Legions that had gone over to the Secessionist cause were annihilated by the rampaging Loyalist Legions. By 975.M33, an uneasy stalemate had been reached, as both sides no longer had the means or the will to further prosecute a conflict that would ultimately end in the mutual annihilation of both the Imperium and the Nova Terra pocket empire. Hundreds of millions of casualties had been sustained by both sides in the conflict, and neither side could afford to expend any more blood or treasure in its prosecution. An accord was finally reached between the two sides, and the Segmentum Pacificus was reluctantly recognised as the Nova Terra Regency, an empire independent of the rule of the Imperium proper.