Interregnum
From Transformers Wiki
The Interregnum is a period of Cybertronian history taking place between the disappearance of Nova Prime on the Ark-1 and the rise of Nominus Prime.
Fiction
2005 IDW continuity
During the Interregnum, Delta Magnus of the Primal Vanguard carried the Matrix of Leadership until a proper successor could be appointed. An Intimate Beheading