No one is quite sure if the Throne Bound King is a Mummy Lord or a Lich or some other type of undead. What is known is that he had originally been a scribe named Prayle. He eventually worked his way up the court becoming the Grand Vizier to the king of the time. He gained so much trust that he eventually became the heir to the old king.
Once Prayle became king, he took steps to ensure that another heir was never needed again. His long life evened out over time. After reshaping Aklatan in his own image, he's spent centuries alternately sleeping or trying to find new things.
The alliance and marriage with The Battle Queen is definitely new.