In the mirror universe, Unimatrix One was the center of the Borg Collective. Backups of the Borg King were kept there. His consciousness could be transmitted to Unimatrix One through a subspace carrier wave. (TNG - Glass Empires novel: The Worst of Both Worlds)
In 2371, the king's exploratory diamond was infected by an Iconian pathogen in the Beta Quadrant. The king destroyed his ship to prevent the spread ot the computer virus and mused whether his next body would be female.