Theridan was a level 90 mythical. He had the highest level among all the past kings. As the king who had founded Themisphere, he was said to have defeated many barbarian tribes and united all the humans into one country. Hence, it wasn't strange that he had a higher level than any sovereigns Jack had met, even when compared to Marcus.
The rest of the kings had levels that ranged from level 70 to 85. Aside from Theridan, another two kings were also mythical grades. Thegromar and Thebalder. The others were rare elites.
Thebalder was Themisphere's fourth king who was a friend to the orcs and also the reason for their country's enmity with the Verremor nation. He was a level 82 mythical grade.
