The period between two reigns

One word substitute is Interregnum.

Lapse: a brief or temporary failure of concentration, memory, or judgement.
Interregnum: a period when normal government is suspended, especially between successive reigns or regimes.
Stasis: a period or state of inactivity or equilibrium.
Anachronism: a thing belonging or appropriate to a period other than that in which it exists, especially a thing that is conspicuously old-fashioned.
Intermission: a pause or break.