The Kingdom of Thyllanor refers to the Fifth through Eighth Era version of the kingdom. Were you looking for the Thyllanorian kingdom, the kingdom of the same region in the First and Second Eras?
The Kingdom of Thyllanor was the title of the land of Thyllanor that included the vassals within the lands of the Karthmeres, and with higher authority than the Lord Paramouncy of the same name. The title was destroyed with the Dragomyr invasion, when the Karthmeres surrendered to the Dragomyrs.
The kingdom of Thyllanor started countless ages before the Dragomyr invasion of Andoras. It's lands rich with gold and it's nobles powerful with influence. You could say before the conquest, that the Kingdom of Thyllanor was the most powerful of all the six kingdoms. What men they didn't have for an army, they could hire. Under House Karthmere , none would dare oppose the wealthy and intelligent nobles or royalty of Thyllanor. However, no amount of gold or men could save the kingdom from the dragons. They learned of the trouble that befell House Dayton to the south of them in the Kingsland, and knew that if they resisted, they, too would be burned alive. After the conquest, they were smart enough to know that defiance to House Dragomyr would be their destruction and therefore, gave up their titles of kings, queens, princes, and princesses in exchange for those of Lord Paramounts.
Potential Second Kingdom
After the Dragomyrs were overthrown, the Karthmeres knew this was their chance to restore what was taken from them centuries ago. Dalan Karthmere II, Adrian Karthmere, and Tyralt Karthmere prepared documents and read upon records to bring back their lost kingdom. They are currently attempting to do this now.