The Interregnum was an event in the history of the Lord Paramouncy of Thyllanor, in which there was no ruling Karthmere (or otherwise) monarch in office for two years, from December 6th, 330 AC to April 16th, 332 AC. During this time, a council ruled for Thyllanor, as a law passed after Richard III's passing that no child could inherit the throne until they were six years of age - usually determined as the age that children are able to know right from wrong, and even then they rule with a regent until they are fifteen. There are laws in place to enforce an interregnum should another child inherit the throne before they are six years old, although the interregnum previously was the only one enforced in Thyllanor's history.
Other lordships may have enforced interregnums, but the Thyllanorian one is most notable. The Interregnum, while it existed, was formerly recognized as a primary branch in the Thyllanorian government.
Reasoning for an interregnum
When Richard III Karthmere inherited the throne in 159 AC, he was barely capable of spatial reasoning or thinking logically, much less in a position to rule a kingdom, even with a regent; who, at the time, was his father, Vicken Karthmere. While Richard III was a great ruler (even, perhaps, the greatest ruler ever in Thyllanor), it was seen as arbitrary to let a child rule when he, perhaps, does not even know he is a ruler himself.