The Interregnum was the period between the death of the old Queen of the Old Kingdom and the coronation of King Touchstone I, her son, two hundred years later. It was begun by Prince Rogirek sacrificing his mother and two sisters, and he and his followers would continue to weaken the Kingdom in every way possible for the entire Interregnum.
As Touchstone was the last bearer of royal blood but placed in suspended animation for his own safety, the Kingdom was ruled by various regents after the Queen's death. However, with two of the Great Charter Stones broken, the Royal line virtually nonexistent, and the Abhorsens slowly lessening in number, The Five Great Charters were deeply harmed and therefore the Charter itself lessened in strength. Gradually, the realm began to fall into disrepair and lawlessness. The final regent went insane and tried to burn down the royal palace in Belisaere in order to free it of the Dead.
By the time of Sabriel, no one reigns, and the realm is nearly post-apocalyptic: the Charter is extremely weakened, Dead creatures roam freely, atrocities such as human sacrifice and child slavery are common, and individual towns, such as Nestowe, are gradually being destroyed one by one.
Sabriel would be the last Abhorsen of the The Interregnum, but it would still prove to be the darkest time in The Old Kingdom's history. Even twenty years later with Kerrigor and his followers gone, Touchstone ruling as King, the Great Charter Stones repaired, and the Great Bloodlines increasing, the realm still has a long way to go to full recovery.