From Lexicanum:
The Nova Terra Interregnum is also known as the time of twin Empires and lasted for nine centuries... Eventually the civil conflict spread to the Adeptus Mechanicus... During the Cataclysm of Souls in 975.M35 the Ecclesiarchy tried to unite the Imperium as a religious state. Their efforts ended in... a civil war unseen since the Horus Heresy.