Dark Ages

The Reign of the Darkest King
History from this time is very patchy. What people do know is that an unpaired Kizier came from seemingly nowhere and became akin to a god. This demigod was named Kiros and he was called many things, the Harbinger of Death, The Wrath of Hell, The Son of the Void, and The Darkest King. Kiros' rule was all encompassing. He took over every known empire of the day, obsessed, it seemed, with every living being on the Planet to be in his army.

His powers, the legends say, were without limit. Kiros was known to arrive at castles that were months away from each other days apart. He travelled faster than news of any rebellion and if he thought there were people speaking against him, they didn't live to speak much longer.

Kiros' limitless power was finally brought to an end, but from here the story muddles again. Some legends state that Kiros was killed, some claim that he turned into any number of things ( a black hare is the most common and the animals are considered bad luck to this day) and escaped. Still other legends claim that he was imprisoned in the blade of a massive sword that could only be wielded by a true hero.

Regardless of the truth of the legends, The Reign of the Darkest King lasted for decades and is the basis of the common knowledge that no Kizier is safe without the balancing force of a Tharii

Other Dark Ages Tales
The backlash of "breaking" the spell created the modern system, as the mage was unable to destroy the bond but could weaken it and transfer it to another mage

Empire collapses into various small kingdoms ruled by mages who style themselves kiziers, people figure out how bonding works over time, becomes formalized into kizier/tharii Pairs.

Some kiziers are not very good rulers, others try but it's just straight up hard especially while they're still figuring out bonding. Stuff goes badly a lot.

People/Plots/Possibilities
Unrelated random fact: the feminine of "despot" is either despotess or despoina, which I read as despotina, which sounds like an iron-fisted princess, and somehow now I want a dystopian princess world. Perhaps during the post-empire dark ages, where a kizier despotina rules in her gravely ill parents' stead and nearly destroys the kingdom. Good times, good times.